I hate to resurrect an old thread but thought I would post back here with what really will be a final update !!
I hadn't run a speed test for a while now and decided to run one just now.
Our download speeds are back up to the 37Mbps mark.
I then ran the test via the wholesale speed checker and it confirms that our IP Profile is now at 38.18 Mbps.
I've no idea what's changed. Maybe the 2 engineer visits and the change to the Openreach modem and face plate, combined with a profile reset has done the trick.
EDIT: I've just checked the logs in our router and it looks like like something caused our connection to drop and re-connect at around 1:34 this morning.