Thank you for that reply. Unfortunately I went into Halfords and as soon as I mentioned three grandchildren the assistant took a sharp intake of breath and said "You are asking the impossible" never even offered to try any of the seats in my car, suggested I buy a bigger car and come back in to discuss it. I said "There must be one make of car seat that is narrower than others", assistant said "Well if you find it come back in and tell me about it!"
ToysRus had a very limited number without Isofix (which I didn't want) so tried one make, which I would have bought but they fitted it in such a way that there was a good 3" of space at each outside edge of the actual back seat thus reducing the space in the middle to about 6" therefore assistant said "Oh you wont get a booster to fit into that small space". I know when I fitted the back facing baby car seats I had about 1" either end of the back seat and about 13" in the middle more than ample to fit a full size booster seat. I don't know how to resolve this problem, but I am sure I cant be the only Gran or parent with this problem