Blimey what would be the chances of LABC randomly turning up to take enforcement action on such things as this........most definitely reeks of a money for old rope scenario!
It can depend.
If the alterations are life safety related (fire, means of escape, loft conversions) then the council are more likely to enforce building regulations - and the time limits are open ended - they get injunctions after 12 months, but still the chances of such deminish over time. They can find out randomly, word of mouth, a disgruntled neighbour or friend - or the seller.
Things like roof and window replacements can be seen in passing, but may be less likely to be enforced against because of time and cost and the benefit of any such enforcement. This type of work will be limited to enforcement within 12 months though, as no injunction would likely be granted by a court. But my local council is proactive in looking for re-roofing work.
For planning issues, again it is visible externally, and the council can become aware in the same ways as above but there are the four or ten year enforcement limits.
What homebuyers don't tend to be aware of is that these policies don't protect them from bad work, only from council enforcement. So if the work is deemed satisfactory, or has been present a long time then the policies are not much use and dont atually provide any security at all.
But more pertinent is the impact of unauthorised work on home insurance policies. Unathorised work, could be by default deemed not up to any required standard and increase risk - thus could void a claim if the insurer was not made of it in the proposal.