That £900 worth of tyres could last up to 3 years. Funnily enough, I was just cleaning my winter wheels - looking them over the tyres are good for anther season so that'll be 3 winters I have used mine. That said, my winter set-up is not staggered and the tyres are not directional, so I have been able to swap them front/rear and side to side.
Have a look at grippytyres.co.uk - not sure if they will have what you want, but when I was pricing up some new summer rears they were £40 cheaper - fitted - than any other supplier/fitter. They were recommended by someone on here, and they come to you. Not used them yet, but will do when I need to.