Thanks for your reply Phil L, the firm that did the job did exactly as you said, as it turned out only two holes where needed as the orig back door opening ( now inside but 1mtr from the new outside wall ) meant they could fill one section from the outside edge. The holes in the outer walls are now almost invisible. I know it's warmer weather now but I'm sat here at nearly midnight having had the heating switched off for two days and its really warm inside, the drafts are def reduced if not gone altogether ! they also replaced the airbricks under the suspended wood floor areas with sleeeved one's and put what looks like a 20ft long loo brush vertically in the cavity between our half and the neighbours ( 1930's semi ) so it didn't spill over into their walls. We paid ?75 and Powergen paid the rest ( or prob the government paid Powergen ) but so far it looks like the heating bill for next winter will be lowered.
Pete