Bugger.
Ok, it sounds like sensible advice - I suppose it'll be better in the long run, than causing more damage. Cheers for the advice, I'm going to have to get used to spending more time on the road bike.
I'll see about finding a local running physio to talk to as well, and will start doing balance exercises.
The wobble cushion I was loaned is a flat-ish inflatable pillow with knobbles on it. It has a valve on the side that takes a football inflator, so I guess its the type that you can vary the pressure. I'll start using this as well.