I now have fitted:

215/35/18 7.5J on the front
225/40/18 7.5J on the rear

I'm concerned about the size of the front tyres which have a small profile and even though the tyres fit the rims I don't like them being narrower and the way they stretch out onto the rims, and how little rubber there is between rim and road. Also they have a different rr to the rear wheels and while not necessarily a problem it's another thing nagging me.

I know the rr of the front and rear wheels is different than the 225/55/16 originals but I'd prefer to have the same rr all round, so would you people recommend changing the front tyres?

i would put 40`s on the front myself.  :y

7.5J rim, I'd probably be inclined to go wider all round.  Or get narrower rims

I was thinking that while I can't stretch to changing to 235/40 all round I could change the 215/35 to 235/40. I would then put them on the rear and move the 225/40 to the front.
The current 225/40 are 200 miles from new and a waste to change, whereas the 215/35 are more worn (not dangerously, just more) and too narrow and low profile for me.

So, I would have the larger 235/40/18 on the rear and 225/40/18 on the front.
The question now is, what tyres... At 235/40/18 do any particular makes/models shine out more than others?

235 was the std width on the ELite and MV6 with 7.5x17, although Vx do over tyre rims in my opinion. Sounds like you have got Astra / Vectra Tyres on those rims.