Will be running a 1.8 blacktop zetec, and have found a Rover 200 series expansion bottle that will fit my engine bay perfectly. Will the Rover and Ford systems run at similar pressures, i.e. will the Rover cap be ok? can't see why not but......

Cheers.
Pete.

No problem Rover caps lift at 15 psi.

However check the little vent in the top of the cap isn't gunged up --- common problem on Rovers in hard water areas --- you could acid clean by leaving soaking in a vineagar solution.