Sounds like the OP bought a part of land from X. X is still their neighbour, so it was a transfer of part - the land belonging to X was carved up. I think this because the OP
Mentions the neighbour not mentioning the covenant earlier in the thread but why would we rely on anything the neighbour says unless we thought they have a duty to disclose because they're the sellers. (They don't have a duty to disclose, by the way).
The case against the original conveyancing solicitor will rest on whether the sol was professionally negligent and didn't investigate the title enough or disclose to the buyers, her clients, the covenant and ramifications which is significant - despite being pretty historic - and fully report to them.
The OP will have to persuade the (in a case against the original conv sol) Court that the covenant IS going to lead to a loss (damages/quantum).
They'll have to quantify their loss somehow.
If they didn't want to go down this route, which will be long, expensive (initial outlay) and arduous then they could sell the house. If they are not in a dispute with the neighbours, they needn't disclose the covenant.
The buyers (previous and future) must due their own due diligence on the property - survey and legal title stuff. Buyer beware still rules, despite recent attempts to up-Chuck that.
The prof neg claim may be mitigated because the buyers did not survey the THING (whatever it is) at all.
They'll have to persuade the Court that had they known about the covenant, they'd not have bought it, despite not having a survey and not actually knowing the costs involved.
First the OP will have to show damages in £'s.
Then they will have to get onto cause.
OP, why not go to a direct access barrister and get an Opinion. It'll sort the men from the boys from the off - they'll tell you what chance you've got.
If you DM me where you arebased, I'll search some out and see if I can get any reccies from my team (back in work next week, but you have time).