It shouldn't ever require the customer to come up with a detail for thermal detailing but in all honesty, it's not really something the installers know about either in the typical replacement window market which is not the market that most people on here are looking at. They "know" about removal and installing back in the same place with new windows. If they also surveyed, they should be allowing appropriate tolerances (usually 5mm all around on a replacement and dependent on opening, it may require 10mm or more). It would be interesting to see the report from Fensa and what wasn't in line with building regulations and why if possible?