Hello matey,
This occurred to me last Friday, get a horrible crunch going into 4th from 3rd sometimes and the same going down from 5th to 4th.
I'm going to be buying a gearbox from Ebay and getting the guys who I use for work to fit it along with all the other bits I will need.
I'm on the understanding that you will need a TL4024 geabox like in this
HERE
I would say roughly if you are looking to get this all done that around the £1,000-£1,100 mark is what you will be likely paying.