The first house that we bought was from the 30s and there were covenants with the builder who had gone out of business years previously.
The issue is whether the benefit of the covenant has passed to the person trying to enforce it see here:
http://www.rics.org/Management/Disputeavoidanc emanagementandresolution/Disputemanagementandr esolution/ledwards001.html
We had no problems nor knew of anybody having any but then we weren't trying to redevelop it or anything like that.
Moving of internal walls is not likely to be objected to but if you start to build an extension someone on the estate might claim that benefit of the covenant had passed to them as a group.
I would suggest you look to see if anyone else of the estate already has an extension. There's nothing to stop you knocking on their door and asking them if they had any issues.