You first port of call is ascertaining that there IS a loss. That the covenant DOES effectively bind the land (despite being positive in nature).
If the covenant does bind the land and that will certainly cause you financial loss, then you have recourse against your original form of solicitors.
The covenant has been missed, so in your position I would be instructing your original firm of solicitors to figure out for sure (whether they have to pay for an Opinion themselves or use their property litigation partner (if they have one) to fully expand on the position. If the overriding concern is that the covenant IS enforceable and you can show that this will cost you £Xxxxxx then you at that point start your claim against your original firm of solicitors.
It may seem counterintuitive to ask your original firm to assess the covenant, but they will be honest about it. If they find it does, then they'll put their PII on notice and the process begins.
If there's a question about the covenant and whether it causes you loss, then you'll be having a fight with them and they'll be putting you to proof with your claim - so you get your Opinion at that stage.
Best case scenario is that the covenant doesn't bind the land (and I would say again, this is unusual for a positive covenant) and your neighbours are bullying you, hoping to outmatch you. It is so rare to bind successors in title -
I hope it's not enforceable and you can tell them to fuck off.
Your original firm of solicitors ought to be helping you figure that out, though, as they didn't disclose the covenant and its potential ramifications to you in their report on title. They also might turn around and say there's no loss here, please instruct your own solicitor and that's that. Then you get your aggressive land law litigator involved.
But find out about the enforceability of that covenant first.