Thanks for the replies all. First thing I wanted to establish was if it was just a problem with this particular box or a more generic issue - so that’s sorted. Been seen before so it’s not
just my cr@p driving.
Second thought on what to do now...
I bought an IL short shift at a good price (unused but missing one bush) from the forum a while ago on a ‘maybe that’ll be useful’ - i’ll try that first and see if that helps with the nice new bushes etc. delrin and lathe to the fore...
Failing that, a mug of tea and look at machining up an external lockout gate between 5/6 and reverse planes somehow. May be difficult as I need to keep reverse operable and want to keep the console even if it’s a track-only car. Will have to rewatch Project Blinky episodes for inspiration.
Failing that, beer and swearing, then fight off the wallet-moths and follow up on the tall shifter
Failing that, learn to drive.... or follow Magpie and use my spare 5spd with a shorter diff because this box might be shagged.

I’m slowly getting used to it - only one grawnch and a baulk last week at Sandown - but very tentative (=slow) on the changes.