Three floor building with two boilers - one meter or two?

The basics: Your domestic gas meter is only able to supply a certain amount of gas to all of your appliance's combined, if the demand for gas is greater than the supply then you have a problem (a very rare instance) But if this is not the case and your gas engineer is ONLY concerned about the gas pipe size to the boiler on the 3rd floor then this is just simply categorised as NCS (not to current standards) certainly no need to condemn the boiler solely for the reason he/you have stated! The solution would be is for your gas safe engineer to pressure test at the boiler gas valve inlet test point with all other appliance's running at full rate whilst this exercise is undertaken, and compare the actual test pressure with the manufacturer's data. Ask him if he has carried out this simple exercise? And gas rate it too for good measure straight after the test pressure exercise.

You do not need two meters, but you do need the correct working pressure, what would need doing is the gas pipe will need to be large enough to feed all the gas appliances. This would be what the problem is, hence him saying he will condemn it. But what the engineer should be doing anyway is running all the gas appliances on full when he services any of the other appliances, so he should in theory be condemning all the appliances, not just the one. He is not doing his job right, get a decent engineer in.