Hi all - I finally am in a postition to get my gearbox sorted - gave it to a specialist in Kent (thanks Dan S and Chris S for your help).
The gearbox has been taken apart, and looks like a synchro spring broke, which then caused the sliding gear to get damaged, which prevented the car going into 4th, and caused a bit of crunching in 3rd.
I need a replacement - the part number is 103060 SLDG Gear 3rd,4th
It is number 26 on the diagram 4-1-2 in the parts manual (the dmcnews pdf has an incorrect part number listed against it)
I can't seem to find this part anywhere - does anyone know where I can get hold of one?