Personally, I think siting the microwave above the cooker hood makes it too high to use comfortably ... unless you're very tall. You'd have to choose your cooker hood very carefully, too - many have the motor in the wall unit above them - so no room for the microwave.
Whirlpool usefully make one that's all one unit:
http://www.whirlpool.eu/app.cnt/whr/en_GB/pageid/pgwpproddtl001/catid/1/subcatid/5/prodid/27897but it hasn't been so popular that all the other manufacturers have copied it ... it's still the only one on the market. The specification says that it must be fitted 650mm above a gas hob ... more if any one ring is more than 3.2kW or the total hob power is above 10kW (you have to ask the manufacturers for the new height).
It would make more sense if you were having an electric hob, because the hood could be positioned lower.
You may also have problems with the gas regulations ... which appear to say that wall units at the side of a gas hob musn't be positioned within 50mm of the side of the hob. Some gas engineers are taking this to mean that an integrated cooker hood must be 700mm wide for a 600mm hob ... and that there must be a 50mm gap either side of a 600mm telescopic hood (when fitted over a 600mm gas hob). Others will fit wall units up to the side of a gas hob ... if that's the arrangement you currently have. It's all a bit of a minefield!