I have just (6pm Monday night) put the line back to the usual socket.
I re-booted the router whilst doing this, as speeds were down again - much better now.
Last night was really bad - often down to 26mbps or 39mbps, with pages being very slow to load.
We only have the ONE telephone socket, and as you can see from the photo, have to use a 'split' thingy to run both phone & router.
I had this conversation with Debbie last Wednesday - the router is not the problem, so no, another one would not help.
To summarise, you have tested another router . filter at the test socket directly and the issue continues. This rules out hardware and also internal wiring and faceplate.
The only option is to arrange an openreach engineer for further investigation.
I'll drop you a PM (Personal Message) with the details.
I'm sorry, but the answer to that one is No!
With everything being tested, the only way forward to resolve this is for an engineer to visit.
Without this we are unable to take the matter further.
Should you change your mind on this, please let me know.