Thankyou all - all comments noted - looking at the photo's provided by Grumblebuns and ExLimey, the situation appears the same - and I agree with Richard48Y regarding wanting to see 1/8" or 3mm - I'm chasing 5mm as left as it is it will be transmitting through the chassis, specifically from the lower indent plug; think I'll re-work the area to give me 5mm clearance.
BDA Quote " If memory serves, the half height nut for the cam was originally a nylon lock nut. If finding a similar replacement is impractical, I would use something like blue loctite on that nut" - yes you are correct; the existing arrangement is temporary as I intend to remove the external "Lotus" indent system for 5th and reverse, replacing it with the original indent and spring system in the 5th gear housing supported by a reverse lock out at the gear lever. (Well - I'm going to try and see how it works)