Well after ordering a steering idler, the GM one on ebay I decided to do a bit of tyre swopping.
My saloon was tramlining a good deal so immediate thoughts were steering idler. So noticed no play in it when I took the wheel off, I ordered one anyway.
Whilst I had the wheel off again today I noticed when rolling the wheel along the drive it seemed to wander a little, just didn't seem right.
So I decided to take the two front wheels off the estate, these are good evenly worn tyres, and I've had no problems with these and they run true on the estate.
Problem solved!

...It was the tyres after all...they are Avon ZV5, if anybody knows what they look like, sort of a weird tread pattern different on each side of the tread.....and crap for my Omega anyway. The saloon runs straight and true now, on any surface. I ended up putting the fronts back on the estate, then swopping the rears of the saloon to the fronts. They are different, they're Nexens and run true like the estate tyres did.

Got a steering idler on it's way, ah well it will come in handy for the future.

Mick