I've been for a quick test ride, a few adjustments later and all seems well.
I think the rear mech was originally set up for a 12-25 cassette, when I put a 12-26 on I never changed anything and got a slight 'rumbling' sound when in lowest gear, now put a 27t on and the rumbling is worse.
I am hoping that it just needed a play with the b-screw to create more of a gap between jockey wheel and cog?? That is what I've done anyway
Just need to pack the bike with flapjack and energy drinks and I'm done