I believe they are obliged to send them given the fact they screwed it up in the first place, this was PlusNet and Openreach's doing, not mine, it was PlusNet that insisted on these visits.
There is most definitely a fault as half my tone range has vanished along with an actual 10Mbps of download speed and my upload speed is still below the low estimate and still dropping.
Being quite frank the cost to PlusNet is of little concern to me, they are there to provide me the service I pay for and have been happily receiving. My clients couldn't care less if my costs spiral out of control so long as I can't hit them with a variation order. Besides, as it was Openreach who screwed this up surely PlusNet have cause to refuse to pay for this service themselves.
I really can't believe anyone would think this is acceptable at all, it's like asking the bank for £100, they take it from your account then give you £75, would you accept that?
As for your comment about the two previous visits, I do agree and actually advised PlusNet it would be pointless as most of the issues I had were in the evenings and no issues during normal working hours so Openreach would almost certainly not find a problem anyway. As already said it was PlusNet who insisted on these visits when my line was functioning at a far better rate and I have that as evidence.