"I bought a house 12 years ago" - 2004
"it had been extended over 30 years before" - 1974
"without planning permission", - was it /were they exempt?
"building regs" - only since 1990
"and because of the length of time there was no problems with the purchase or getting the mortgage." - this was at the height of the boom when banks would lend on anything! It is entirely possible that your solicitor "qualified" their Certificate of Title to say that the extensions were not exempt but that no enforcement notices had been served.
"I recently had it valued and was advised by the auctioneer that these extensions now require retrospective planning permission before they can be sold. " - talk to an architect or an engineer filling in the gaps and see what they have to say.
If you are not thinking of selling or re-mortgaging, you have time to take considered advices and plan your next move.
If the extensions are not exempt ( and I suspect they are not from what has been said) you should consider applying for retention permission - per kceire's post - it cleans everything up nicely, if you get it.
Per moneybox
"Isn't it possible to get an Opinion on Compliance from an engineer which is accepted by solicitors?"
If its not an exempt development, an engineer can't certify that it is!
mf