A restrictive convenant may be in favour of a named individual or the proprietor of the land title. Either way removing it legally isn't cheap - it may be done by applying to the Lands Tribunal using the case that the convenant serves no useful purpose any more. You would probably have to employ a specialist (solicitor) to assist you.
If you can't find the holder of the convenant (because they are maybe deceased) you could go down the route of indemnity insurance. The proemium will depend on the perceived risk by the insurer and is impossible to guess. On something as old as 1928 likely to be very small premium.