Tools & Service

 I'm not familiar with the term "Breeze Block", but if that means a standard concrete structural building block the the answer is NO!
While it would actually work, it would be a long, slow, tedious process that would certainly shorten the life of the drill.
As a sparky having worked in concrete accommodation blocks (walls, floors, ceilings) for the last 6 months, then you need the biggest hammer available (i.e. the GBH4) for chasing.  I've also tried it with both a GBH3 & a GBH 36VF Li, with limited success, a lot of noise and slow progress.
Let's face it: chasing's not a really pleasant task.  Anything that's going to speed the process is good.
I've also tried a Max hammer.  Fantastic speed, but neatness is badly compromised, and I've even accidentally displaced the odd brick!

Now I see where you're coming from Ian.

20m is an awful lot of chasing.  It appears breeze block is similar to or the same as what I guessed it might be: i.e. fairly dense and hard.

Given that fact, and your explanation that it's a one-off exercise, here's my take on your requirements.
The GBH 2-20D will be hopelessly inadequate for your task.  However, a GBH 2-28 DFV would be capable, and while not the most suitable alternative (I'd still recommend a GBH 4 or equiv.), it would still better fit your criterion of a "Multidrill" that is eminently suitable (having interchangeable chucks) for a variety of general drilling duties.  Unfortunately it's also significantly more expensive than the drill you first mentioned.

Once you've used an SDS machine for masonry work you'll wonder why you even considered a hammer drill at all.

There's alternative quality drills available for a similar price that would also fulfil your requirements, such as Metabo's UHE 28 (now upgraded to a new UHE 2650 model) that have both chucks, chiselling & drilling functions, but have the additional advantage of another "second gear" for high speed drilling, and slightly more power available.  Having used the former of the mentioned Metabo Multihammers, I would also suggest that it makes a better drill than it does a hammer, and vice versa for Bosch. Milwaukee also do something similar, but I haven't used one since their Kango days.

Without doubt, Bosch currently make the best range of hammers available.

When chasing, try to use the mortar joints if possible.  They're not only softer and easier, but assist greatly in keeping you on a straight course.  I'd recommend a narrow masonry gouge for cable installation, preferably with wings (to provide depth control) if you lack confidence &/or experience, which are available from the Bosch online accessory catalogue.  You can fix cables flush to the surface or even semi-recessed with cable clips hammered into the vertical mortar joint intersections.

 

I did start considering the new GBH 2-24 for the extra power, and the free bits in the current promo would offset the higher purchase price, though I may be allowing this one job to dictate too much in this purchase decision.

The smaller size and weight of the 2-20 still has a lot of appeal for the long term. I lifted one off the shelf in a local store and it's about the same size and weight as my old impact drill - I could just about manage that one comfortably when I needed to use it one handed, so in an ideal world I would be choosing something even smaller. The larger SDS drills in the store (might've been 2-26 or 2-23?) defintely felt noticeably more unwieldy and heavy.

If money were no object then I would get one of the 14.4V or 18V compact cordless SDS as well as a larger GBH 2-28 or 3-28. Anyway, some good info in the above replies, so thank you very much for the input, I'm getting closer to making a decision now.

 Perhaps the term "hopelessly inadequate" is a trifle uncharitable.  Please substitute with "frustratingly inadequate".

Ian, you have expressed the desire to chase in structural concrete blocks, with a potential density of up to 34 MPa.  I shall once more attempt to dissuade you from the expensive mistake of using too small a tool.  

The GBH 20 has a quoted max impact capacity of 1.7J.  This is the same as my GBH18 cordless hammer, a tool I principally use for setting 5mm. anchors.  The tool will also drill adequately in larger sizes, albeit slowly.  But its the ideal tool for my stated purpose:  some 6000 anchors installed in walls and suspended slab ceilings over the last 6 months.  It's the ONLY tool that I can confidently and safely use overhead and one-handed atop a ladder.  A tool with such a relatively meagre "punch" won't adequately chase in 34MPa. concrete.  Makitas, at 1.9J are equally inadequate for the task.

My GBH36VF-Li (2.8J) won't adequately chase either.  It's best for 12-18mm holes, and will chisel out the backs of electrical blocks into masonry, where too much "punch" would dislodge or loosen fixings through fracturing.  I have used it for chasing, using wide and narrow chisels, and gouges, but it's a slow exercise. To make a living at this game, one can't afford to be too slow!  This drill is woefully slow.

My GBH3 (3.1J) is the ABSOLUTE MINIMUM required for me to chase in structural concrete.  In other respects, it's power and handling characteristics are similar to a 36v cordless.  Lighter weight, and a superior speed control (trigger) mechanism make it in some ways a superior, but less versatile, tool.

The GBH4 (5J) is just a better tool all round.  I don't notice any meaningful difference in weight.  I'm getting on now (52), and aren't as strong as I once was.  Nevertheless, this tool is light enough and sufficiently easily handled to safely use atop a ladder and at arm's length.  What is more important is that it does a better, neater, faster, safer and cleaner job of chasing than all the rest.

I have 7 SDS & Max hammers of my own, and access to my employer's range of Hilti drills.  I have been renovating 7 multi level student accomodation blocks at my local university: over 150 rooms have required rewiring, all in structural concrete floors, walls and ceilings. I have drilled thousands of holes, chases, penetrations and anchors for plumbing, electrics, steel cable tray, reinforcing, access, telephony and data.

A GBH20 won't do the job. End of.  A GBH24 (2.7J) will struggle to perform adequately.  Hire the flamin' tool if you don't want to buy one.  But if you want a multidrill, then there's really only 2 choices:  GBH3 or 4.  All the others are little better than toys in 34MPa. concrete.  It would be quicker, cheaper and better for your health to get a quality masonry chisel & club hammer and chase by HAND than try to use a 1.7J machine.

As for all the other nonsense about L boxxes, accessory chucks etc...  that's just what it is.  Nonsense. Cutting to the chase, as it were; what do you really want? A new hammer drill or a task accomplished?  Put some high-Moly graphite grease in your old drill's gearbox, available from any parts retailer for a few pennies.  Buy your GBH3 or 4 from Lefeld if you want a fancy L boxx. Or hire one.

Just take it from someone who knows.  You can't do decent work with a toy.