It happens on more modern estate developments too. A builder developer of a largish estate will put a convenant on that lasts say 5 years to stop owners modifying the houses without its permission before it has sold out the rest of the estate. The original owner won't want it demolished but may want a smallish fee for agreeing the permission retrospectively. If you can find the owner - if you can't an indemnity policy will do the trick. Either of the above solutions is likely to be cheaper than trying get the convenant removed.
If you took the external doors away from where you built the conservatory, I hope you got Building Regs approval? - otherwise you may have another issue to resolve.