I had a very similar issue with Talk-Talk (one of the many reasons I am no longer with them).
An ADSL connection that was supposed to be "up to" 8 Mbps would frequently drop out, or fall back to very low speeds (double-digit kbps). An "engineer" was booked, didn't turn up on the day, a second "engineer" was sent round a couple of weeks later...
The first thing they do is to pull out the phone extension that I had (properly) crimped into the master socket using Cat-6e, despite being told, and shown, that their shitty modem did the same thing if plugged directly into the master socket, and that the fault occurred every time there was heavy rain (obviously some sort of problem in a junction box in the street somewhere getting flooded).
Cue an actual engineer, some weeks later, actually visiting the said junction box, and the issue going away. Probably someone from Openretch, since the copper is maintained by them.
Then a bill for £50 for the "engineer" they sent round, who I hadn't asked for but had been told was a necessary visit before they'd investigate anything outside the premises, and who had managed to do nothing except damage.
It took me about 40 minutes of arguing with someone in an Indian call centre before they finally agreed to cancel that bill and refund me the £50 they'd taken from me on Direct Debit.