It does depend what regs you are talking about .... You don't HAVE to have building control sign-off, but without it your buyer doesn't have the assurance that all structural and safety protocols have been complied with. Firstly, it might be wise to find out specifically whether it was just that the paperwork wasn't filed - in which case you can probably get a 3rd party or the Council to help file and complete if there are any photographs of the work (most builders do this!) before it was covered up - or whether actually the work doesn't meet building regs. If it doesn't meet building regs, then you may need to do some work in your home to comply.
Having said all that, it is up to your purchaser whether they are happy to proceed without as like I say I think in private dwellings it is the owners' risk.
It's always worth pursuing someone if it was their responsibility, but I suspect that you have little legal recourse if the company no longer exists. Sorry!