Mega thread bump time - now that the weather is a bit nicer I've finally got round to sorting the vibration issue that I mentioned in the first post, so I thought I'd update this thread.
I'd been through the basic service stuff to check - the coil seemed ok & there was no misfire, so didn't think it was that. Plugs seemed fine too & I couldn't fine any vacuum leaks either (I'd already changed the PCV hose & valve) - the car was fine when driving, but whenever it dropped back to idle it became a shake shack again - and the electrics made it so much worse.
Anyway, I changed the engine mounts at the weekend & it's fixed the problem completely - the car is now super smooth at idle & feels a lot better to drive too. Had a bit of a mare with the underneath mount as one of the bolts was really rusty & snapped, but other than that (had to sweet talk a mechanic buddy of mine to sort that one), not too bad a job.
So, I'm a happy Focus owner one again