Welcome to the forums @charistset . With respect, you are jumping to conclusions by blaming Plusnet - the fault more than likely will be in the BT infrastructure, which PN have no control over.
I would suggest you check out the suggestions in this topic:https://community.plus.net/t5/ADSL-Broadband/ADSL-Speed-faults/m-p/1270245#M297501, and in addition do a 'quiet line test' - dial 17070, option 2 from a landline phone and make sure there is no noise on the line - if there is, report a phone fault first, your broadband will not function correctly if there is aphone fault.
These problems can occur at any time, even if one is 'fixed'. there is no guarantee that it for ever.