Just take it to your local tyre fitters (Kwik Fit, ATS, whatever), and they will give you an all in price. Or phone around for the cheapest price.
That's what I was trying to say Jocko. Just ring around a few local tyre fitters, the OP should find someone who will fit the correct tyre for around the same price as the mobile fitter, and will not charge extra for fitting a single tyre.
Mobile fitters have costs coming out to you, and they have probably worked out their fitted tyre prices based on fitting pairs of tyres. If you only have one fitted, they might have to charge you an extra fitting charge to make it worthwhile - otherwise, that may well be any profit gone. There's not a big mark up on fitted tyres.