Finally plucked up the courage to remove the adsl filters and install the MK3. As Dray helpfully suggested I left it turned off for 30 minutes while I fitted it. Its reconnected now and is looking slightly better in terms of the noise margin:
Before (remembering kbps is actually mbps due to a bug):
After:
I haven't seen the downstream noise margin below 8.5 since our speed was banded to 32.4mbps.
I didn't expect the downstream sync speed to increase as I'm pretty sure we're banded at this point, but at least its no worse and the noise margin is less!
Our next door neighbour recently had fibre activated too, and hes apparently only getting 16mbps down! My 32.4mpbs doesn't seem so bad at the moment. Will be trying to help him.