I would think that he was probably badly advised by whoever supplied his conservatory, possibly through ignorance, but I think that because selling conservatories is very competitive, some companies probably don't tell householders that they need planning permission as of course it adds costs. They should do really as they ought to be aware about planning rules. You can do certain things under Permitted DevelopmentRights but you can't keep adding with out PP unless you are still within what is allowed under the PDR for your house. Not all house have PDRs, most new houses will need planning permission for virtually any external changes and even for garage conversions. I'm not sure why they want Cliff to demolish his as surely all he needs to do is to apply for retrospective planning permission. maybe he did this and it was refused, if so an appeal is the next step. When all else fails the LA enforcement departments will rarely resort to demolition these days as they can only really afford to send out written notices.
AJ