Estate Rentcharges

Hamherb

Hello - I'm new to the forum but would appreciate any thoughts on the nightmare of estate rentcharges.
My siblings and I own (freehold)  my late mother's house which, following completion of probate, we have attempted to sell.  The house is a modest terraced house on a small development (about 20 years old) of mixed flats and houses with a management company responsible for maintaining the common areas. We had a buyer whose solicitor then found a rent charge on the property, something we had never been aware of and, to our  knowledge, neither had our parents. After 5 months, the buyers were advised to withdraw which they subsequently did.  Whilst trying to resolve the issue by Deed of Variation (and that's another story!) we discovered that the original Rentcharge document only listed the original purchaser who owned the property for a couple of years.  Our solicitor has advised us that the document should name the original purchaser but also to add "successors in title", i.e. all previous owners and now us.  This has to be agreed by the Land Registry and it has sat in their in-tray for over 2 months now.
Does anyone have any experience of such a situation or know how long the LR may take to resolve this.  Our solicitor has told us that it is not even a certainty that they can alter this document in which case, the house is apparently unsellable until 2037 when rent charges can no longer be imposed.  It seems that mortgage lenders are very wary of these charges.
This has been going on for over a year now and we're very despondent about how to break this deadlock
Thank you