Recent fibre install - dropping speeds

[#1]

Hi,
I had my fibre connection installed and activated on Thursday. I checked the speed after install and I've been seeing a progressive decline in performance ever since. Results from your speedtest checker are:
Today 21:57 19461 kbps (2.43MB/s) 13267 kbps (1.66MB/s)
Today 19:06 30221 kbps (3.78MB/s) 15728 kbps (1.97MB/s)
Thursday 17:57 37711 kbps (4.71MB/s) 17531 kbps (2.19MB/s)
According to the BT Wholesale speed checker, my line is configured for 76mbps.
I've done all the usual stuff - reboot router, modem, PC, virus scan etc etc.
Can you check and fix please.
Thanks,
Mark

[#2]

Incidentally, the BT Wholesale test page now appears broken. It runs the TAP test and then reports:
Test Error
You appear to have not logged on as speedtest@speedtest_domain. In order to run the next test you should login as speedtest@speedtest_domain.
Please wait while you are redirected..
(and then doesn't redirect)