Thanks for your reply Taily.

I don't think that reverse is affected, but that said I can't now be sure I actually tested it at the time it was playing up.

Makes a little whine as I slowly get under way (slipping) then a shudder (on most occasions) and then I feel it engage and power goes through as normal.

Yes, checking Oil when hot and idling in Park.

Box had been rebuilt about 2 years ago (not by me). I've had it a year. Early on had a leak at front (pump) seal and converter was found to have defect on seal surface so put another second hand one in. Put new oil in it then but no filter change.

Yes, will probably have to take it to a shop to get an opinion. Just wondering if anyone else has had similar and recalls what the issue was.