Buying recon gearbox for S3

Only 2nd post so still a bit green......I have a Ex-MoD LWB S3, 2.25. It jumps out of reverse, and 2nd and 3rd when decelerating. I don't have the skills or experience to rebuild it myself so I'm looking for a recon unit (preferably rebuilt with OEM parts). I've trawled the net and eBay and prices vary from £250 - £550. I tried to pick up on the earlier 'Engines and gearbox' thread, and someone mentioned Witham for engines, do they do gearboxes as well? If not who would you recommend and what price should I pay? Oh and the gearbox is fitted with (Fairey?) overdrive...should this make a difference?

Apologies if this has already been covered