In reply to a post by Chrysalis:

a not high enough tcp buffer will cause slow per threaded speeds however if this is something that wasnt happening to you before then it would seem thats not the case.

Well it's not that (I can't think how the TCP buffer size would automatically change itself, getting smaller at 7am and correcting itself around midnight).
It's throttling, pure and simple.

I have now got fibre and am experiencing the problems you are experiencing too. It was actually all fine for the first week but since this sunday I have been getting inconsistent slow throughput in the evenings.

Maybe there is a problem at the Shrewsbury exchange?