supply would be circa £300per sq m cant comment on fitting costs though as it would depend on a lot of things.

Replacing single with dg is not normally cost effective in its own right.

Have you considered secondary glazing or other insulation options?

Tony

What spec are you going to provide for your £300/m2?

I'd charge more than £600 /m2 for English oak but, sorry, I don't do office blocks :)

The dg unit itself, (4-16-4 Pilk K argon fill, insulated spacers) without frame, costs about £50-£60 per m2.

Triple glazed soft-coat Scandinavian-style timber windows being quoted at £300-330/m2 delivered, presumably less for a whole office block. The cost of triple over double is so small as to make it a no-brainer - but the choice of UK suppliers whose rebates are deep enough for 4-12-4-12-4 units is few:

Russell Doortech, manuf'd in Glasgow or by Norwegian associate, 0141 958 0444 (whose quote I have recently accepted)
Tanums (import) 0115 9321013
SP (import) 01347 825610
The above three are in that price range; the following are more:
Sashless, manuf'd in Yorks to a design developed at my request 01609 780202
Green Building Store, manuf'd in Yorks, the eco bees knees but very expensive 01484 653765

The absence of several household names, who can still only do double glazing, is striking , incl. Rationel (import), Eco Merchant (Vrogum import), Scandinavian Window Systems (various import), and giant empire Inwido who have swallowed the once-excellent Swedhouse. The the majority of suppliers included are UK manufacturers in also surprising - Howarth, of Air-Supply Window fame, have a design under development too. A few other companies blew the invitation to quote in one way or another, so there may be others.

Other companies e.g. Vrogum and Inwido do even higher-spec 3G windows with 4-16-4-16-4 glass and frames timber/cork laminated frames, for European-market Passivhaus etc use. And there's Canadian sources, esp. interesting glass-fibre framed - Paul in Montreal has the info.

There are other ways of doing triple glazing e.g. Velfac's aluminium outer/timber inner stuff, which can be made up into slim-sightline facade fame assemblies - maybe of interest for office block?

Don't give me that, Biff - as a supplier you'll have answered that to your satisfaction - what's your conclusion? I just looked at manuf's tables and for the small extra cost of 3G, within total window cost, the improved U-value was disproportionate. Two points conveniently mislead here, allowing backward UK manufs to stay complacent:
1. It's true that squeezing skinny 4-6-4-6-4 3G units into a small rebate designed for top-notch 4-16-4 2G units is counterproductive - more cost, less performance - unless you're paying for Krypton fill anyway, in which case 6mm gap is optimal.
2. For windows expecting major insolation, 2G may give better whole-heating-season performance than 3G; but a) I'm unconvinced; b) how many windows really truly get that much sun, and the building configured to take advantage of same?