90 day notice - if in the terms then that is the contract and comes down to legal arguments over what forms a reasonable attempt to provide a service.

Some business providers would have fought those charges probably before they even got to you, e.g. AAISP certainly if the internal wiring is shown to be fine and the issue is not REIN created by yourself then no reason for them to have been raised and you should have been disputing them since the first charge was raised.

Some providers simply pass on the bills and don't actually look too deeply into whether the charge was raised correctly or not.

Take Daisy to the small claims court. Doesn't cost much (£30?) but claim that they are charging you extra for a basic fault in their service. Most times for small-ish values companies will just settle out of court as their expenses for challenging would be greater than their loss.

Put it in writing and make sure you send it to them with a copy of youe letter.