I'm posting this incase it is of help to anyone else.
I had a problem with my bRAS profile not updating, despite >3 days stable sync speeds (see this thread http://portal.plus.net/central/forums/viewtopic.php?t=44601 if you're interested). Whether coincidence or otherwise, after I posted the above thread my bRAS profile reported on the portal increased back to 1500kbps (and I think a thank-you to Phil is in order as I assume he intervened because my profile changed mid-afternoon, not at the automated 8am/8pm - so thank-you Phil!).
However, both the portal's speedtest and several others still gave the ~980kbps download speeds that I'd be stuck on before. Tried again after 8pm yesterday and after 8am this morning, just incase, but still only ~980kbps.
However, godsell4 suggested something to me a few weeks ago that worked for him and I'm really just repeating it here so others can see if it does anything for them.
I ran a BT speed test (reset your router log-on to speedtest@speedtest_domain with no password and run the test by putting http://184.108.40.206 into your browser). This gave a result of 1456kbps.
Immediately changed router back to normal log-on and re-ran the portal & other speed tests and wey, hey back upto ~1380kbps.
So, immediately before I ran the BT speed test I had a reported bRAS profile of 1500kbps but only getting ~980kbps. Immediately after running the BT speed test all is sweetness and light at ~1380kbps. Just running the BT speed test seems to have corrected the disparity.
Maybe if anyone else has a seemingly stuck download speed compared to their bRAS profile, then they might like to give this a whirl too.