I need help! (Not professional-I've had that-it doesn't work! )

The overdrive on my Mk2 estate works fine when cold, but once the car has warmed up it refuses to go in! On the rare occasions that it does go in I get a 'clutch slipping' feeling?

I know nothing about these gearboxes so any suggestions would be helpful.

The gearbox oil was checked before the HCR and the car hasn't been used much since then. I will be checking it again this weekend, but wondered if there's anything else I can check whilst under the car?

Thanks in advance.

You could try cleaning out the filter. This is inside the rectangular alloy cover at the bottom of the OD unit held on by six bolts.

Hi, You say oil was checked, was it drained and refilled or just the level checked?
If only level checked I wonder how long the oil has been in there.
Maybe some fresh oil would be a good place to start.

well, seeing as it was a nice evening (and that there was bugger all on Tv) I decided to have a look.

The oil level was fine (In fact it started running out as soon as I undone the filler)

So I undid the sump. The filter looks fine and the rest of the parts looked clean abd grit free too. I will give them a proper clean tomorrow and refit them. (Although I need a new gearbox sump gasket-any idea where I can get one of these?)

The oil didn't look that great I have to say, so maybe I'll drain and replace anyway.

Then maybe check the solenoid out.

Just one other thought.
I had all sorts of hassles with my S's overdrive either not engaging or dropping out by itself.
Turned out to be the gear lever switch.
All my o/d problems have been electrical, loose or dirty connectors or bad earths.Or bad switches.
I've put relays in all my overdrives. Probably not really necessary on J types but I then know the little switch on the gear lever is only carrying relay coil current.
J type solenoids either work or don't work.They don't come apart. Good luck, I'm sure it's something simple.