Check this first, David............
https://interactive.planningportal.co.uk/
Conservatories are treated just like any other extension. You may come under "Permitted Development". The above guide should help.
If you think you might come under PD, then you can apply to your local Planning Dept. for confirmation via a "Certificate of Lawfulness". That should satisfy the freeholder.
The question of whether they were "informed". would surely show in their records. After all this time, and crucially, with no objections from anyone having come to light, then I'm sure there is a way forward.
I've bought a Freehold in the past. It's usually do-able.
Because of the doubt over whether they were informed in the past over the alterations, I would strongly advise using a specialist Property Lawyer. I would expect your own local Solicitors Practice to have at least one.