Often these back boilers once serviced and tested by a Corgi engineer can be nearly as efficient as a new boiler, and they are probably more reliable, as they are not dependant on sophisticated electrics to control them or go wrong.
You could have the existing one fully serviced and see how it performs, or pay to have it replaced with the one you can get that is 2 or 3 years old, Corgi engineers can replace these subject to the conditions, which it looks like you will meet, that I have found for you, and copied below.
You were right; it seems he didn't know what he was talking about, and tried to get you to accept the option that was easier for him, in order to make more money, do not get conned, get another reputable Corgi engineer who understands the rules.
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Building Regulations allow for conventional back-boilers (ie gas fires with boilers, fitted in fireplaces) to be replaced like-for-like in situations where it would be difficult to fit a new condensing boiler against an outside wall. The exemption formula is worked out on a points basis, with 1000 points being required to allow replacement with a similar back boiler. A mid-terrace house starts with a points score of 640, and a flat with 710. Moving the boiler to a different room with an outside wall (usually the kitchen) incurs another 350 points. If the property has cast-iron drainage pipes - which would be damaged by the acidic discharge from a condensing boiler - then it would be necessary to install a soakaway at ground level to accept the condensate, and this adds 100 points. Should the new boiler position require a flue longer than 2 metres, this adds another 200 points. So in many cases the 1000 points total is easily achieved, and a replacement conventional back boiler would be perfectly acceptable.
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