The conveyancers who acted for the purchasers. They have the greatest moral culpability and there are other legal principles at work that suggest they should bear the loss. There are numerous difficulties and obstacles, though.
N.B. Throughout this post, I assume the conveyancers did not explain the ground rent escalator clauses properly, spelling out the grave effects, to their clients. Doing so properly would satisfy their obligations to them. The criticism only applies to a conveyancer who did not do so.