I have just replaced my gearbox(a different problem). I got a used gearbox for £88 inc. vat. I also had a price to rebuild my other box and that was £295 + vat with me taking out and putting the box back in! Its obviously a risk getting a used one but a lot cheaper. You need all the right tools and preferably two people to do it.It also takes a fair bit of time. Hopefully you will not need to do any of this though!