Recently bought a house with a loft conversion (done in 1993) and no building regs completion certificate. After checking with surveyor (his opinion was that the conversion appeared to be in order) and our solicitor we asked the vendors to purchase building-regs indeminity insurance policy (approx 50GBP) which we can pass on to whoever we sell the house to and so on.
But we only purchased the house because the surveyor was happy that things seemed to have been done properly. If he wasn't we wouldn't have bought it.
Rob