Car all ready for MOT ..... and it wont turn over, just a loud 'click' from solenoid, all connections & earths checked and OK, so time for a replacement..... but struggling to get correct unit, it appears as though there are 5 variants for the 2L and I'm struggling to get correct one (Lucas), now awaiting 'special order' delivery from europarts 🥴

Check battery first, could be flat. 

New battery, fully charged, when i left ignition on and put jump lead from +ve side of battery to +ve feed to solenoid and car turned over and fired up

Probably too late -- but refurb of the solenoid is quite easy -- it will be burnt contacts which need cleaning.  Just be sure to un-solder the coil wire before opening case, should be worth doing as a spare unit.  

Now sorted....... turned out to be faulty push start switch in gear knob ..... new circuit from ignition switch to starter solenoid through 4 pin relay just the job