I'm still assessing this, since I changed my shift lever/tube. Not too long after this the car seems to sometimes pop out of third gear sometimes, possibly on over-run is when it slides gently out so you're free revving when you want to carry on.

This gearbox was a nice rebuild by Rick Pembro some years ago. Does anyone know if its an easy fix if it's a fault?

Is the lever on the gearbox in the right postion, it might be rotated??

Did you mean the actual shaft going into the gearbox? It hasn't moved at all by the looks of it. I'll have to double check though.

Most likely, it's jumping out because it's not fully going into 3rd gear.

So look for slack/mis alignment in the mechanism.

Roger you're very clever. It was sort of that I think.

Around the same time as this started happening, I had pulled the rubber gaiter thing that's on the gear shaft on the gearbox, nice and snug. It always just pops off normally.

That seems to have been enough to start to nudge it out of gear after a bit. It hasn't done it again since I unsnugged that gaiter.

I drove it a couple hundred miles to Kent and back today and no problem. Was a joy to drive actually.

Can't say I'm clever! Probably put the anal in analytical though

I forgot to update this thread. The solution was it needed a new detent spring. I opened the gearbox lid and actually didn't find one in there. This was odd. I didn't see it ping off, but who knows. Put a new one in and sorted. Very easy job.