Hello Sam,
5th gear problems can be associated with the gearbox itself rather than selectors or linkages etc. On some makes it is possible to effect repsire to 5th gear issues without actually removing the box - dealt with by removing the end cover.
What part of Wales are you?
You have "John the Box" near Swansea who has a good reputation. Not sure the actual trading name but should be Googleable under gearbox repairs Swansea.
Hope this helps.
Kind regards,
Gareth.