MOT Services Halstead had Bob back and and (bless em) spent most of  a day examining every bit of the gear linkage trying to work out what’s wrong with it and how them fitting one bush could have caused what has happened. 

They’ve put me in touch with a specialist garage (or I could go back to the other one who tried to sort out the engine before) but it’s a fair way off and at the moment I can only find 3rd and 4th gear & reverse, so I wondered if there was anything obvious that could be causing the problem before I attempted to drive the 20-odd miles on all sorts of roads with only two gears