@Simon01 Any advice on this - anything that can be done to get the router back up to speed?

  @David-TP  Many thanks for the reply.
I've had the router 4 years, origionally on ADSL  but moved  to VDSL  / FFTC  in 2018, where performance started ~27Mbs, rose over the following months to ~32Mbs, gradully tailed to and held at ~26Mbs into 2021, and has since slipped further even as low as 17Mbs - but has been ~20 for seversal months. 

Have had engineers out including last week, and when they have tested have shown the line to be good for at least 30M, indeed when I put a talktalk routers be that HG635 'super router' or their 'WiFi Hub' routers  in place straight back to Max rates of 30-36M and Actual rates of ~30M !   Note in all cases from the BT filtered Master socket.
When I put the VR back in place it shows a Max rate of ~34M but as shown Actual is a lowly 20M. I have tried with both SRA enabled and disabled, however with it enabled the Max rate is only ~22M.  :-(

It is almost as if with SRA it is remembering an 'old' possible worst case and is stuck at that.

Obviously dissapointed as the TT routers give a 50% better rate  (but fairly useless as a router with a server / port management etc behind them).  :-(
I feel that perhaps a way of improving this is to try and use a TT router as modem/bridge and bypass the VR's modem just use it as pure Router :-(

  @David-TP  Hi David,

Hmm, there is one difference that I can see, which  is that the snr on the VGR2800, sticks at ~ 14db, where as the TT routers happily work on around 6db, so I think part of the problem is the VR one is not maximising  the throughput due to being stuck holding an unnecessarily high snr hence worse / lower throughput,  but why? 

What can be done to get the VR to retrain itself to a similar snr value that the TT routers are happy with?

On being only 4 years old are you saying that TP-link routers only perform better than free ISP ones for a year or so, surely not ! ?

If I bought a new TP router your suggesting it would as well as or better than the 'free' ones but only for maybe a year - this does not make very good sense?