@helandpete Welcome to the forums. Plusnet staff support, like the phone service, is still severely restricted. If you would like help from the community, a good starting point is to have a look at https://community.plus.net/t5/ADSL-Broadband/ADSL-Speed-faults/m-p/1270245#M297501 and supply as much detail from it as possible.
It would also be a good idea to carry out a 'Quiet Line Test' - dial 17070 from a (preferably corded) phone and select option 2 - there should be no discernable noise. If there is, report a phone fault first, as without a clear phone connection, your broadband will not perform correctly.
If you reply to this topic with as much detail as possible, there are plenty of experienced members who can advise on possible remedies.
I appreciate I am not staff, but in the current climate, you are more likely to get help from fellow members.