Thank you for all the, helpful comments and suggestions, @jublybug
Only ten days and everything is done bar a tiny bit of adjusting. 1st, 2nd and reverse are a little notchy but all gears are present and correct. I had not realised that there was a flat on the tower shaft and selector arm. Had I realised I could have centre-popped the life out of the arm and fitted the new one at my leisure. As it turned out my new arm arrived this morning before 10.00am and was effectively done in half an hour. I did centre-pop the flat so I could see it and it did give a bit more resistance on assembly.