I have been reading a lot online about 5HP24 and I am none the wiser. It still looks to me that it is more likely major mechanical issue that needs gearbox to come off for repairs. I will do some driving and logging early Saturday morning while there is not much traffic to interfere and it's cooler so that not to overheat box with torque converter slipping in 5th.
On my journeys I have discovered this place
https://www.planetgearsuk.com not far from me which offer some other 5HP24 boxes like in BMW and Land Rover for as little as £350 to rebuild of box +cost of parts required or sell reconditioned boxes for under £900
https://www.ebay.co.uk/itm/RANGE-ROV...IAAOSwVvRbK5IP. Have not seen any bad reviews but a number of positive with quoted costs about £1k for remove, repair, refit, which would be just about my upper limit. So after I gather more data I might send them details and see what they say.