Here's a definitive summary for you (that also corrects various bits of mis-information above).
There was a requirement for Building Regulations approval for this work back in the 1980s.
The BR approval doesn't 'certify' the property as a 3 bedroom house - it establishes that the conversion was designed to a suitable minimum standard and that the design was followed reasonably correctly (this is done by inspection).
In theory the BR department could demand retrospective action to demonstrate the structure is sound back after 20 years are unlikely to do so - as you have been told.
This is NOTHING whatsoever to do with Planning Permission, as some bright spark above suggests.
Technically it was missold, but after 3 years your opportunity for action is limited - what are you wanting to do? - make a civil claim for damages against the estate agent?
Because it can't be claimed as a 3 bedroomed property, as the attic room should really only be described as 'storage space', that impacts the free-market valuation - however a valuation is only an average opinion by a professional of what someone would pay for it - you thought it was worth what you paid for it, so why are you worried?
Your solicitor should have found this out - it is part of the standard enquiry sheet solicitors ask sellers to complete. There is a question something along the lines of 'has the property ever been converted or modified and if so please provide copies of planning consent and/or building regs approvals. Your best recourse now is to write to your solicitor requesting he/she extracts the files from archive to check for this point. Either the seller lied on the response form or your solicitor was potentially negligent if he failed to ask the question.
But taking legal action now on the basis of this may not be something you wish to do - it costs money and there is no guarantee you will win.