I've come to the conclusion that (one of) my massive oil leaks is because of the O-ring on the end of the pressure relief valve. I've seen a kit with a circlip and O-ring for 20 quid on eBay which sounds like a bit of a pisstake. Does anyone know what size the O-ring should be, offhand?

I think 19mm x 1.5 if memory serves me correctly.

I have a bag of them at home from when I did my engine rebuild (though I'm not at home to check it!)

Or, I get back on the 19th and can have a look to clarify if needed.. I might even have a couple of spares I can send you, if you don't have any by then.

I'll have a rummage at work and see what I can turn up. They're only pennies online anyway, which I suspect is why you have a big bag of them too - "oh, no point buying just a couple when 50 is only a pound more..."

It looks like it ought to be doable once the radiator hoses are out of the way.

You got it.. I think the minimum order I found on the eBay listing was 10 of them, for a little bit more than what someone was selling 2 for...

It should be possible to do in-situ, as long as the end cap isn't stuck in and doesn't refuse to budge and come out (though if it's leaking, then it should come out ok!) On one of my front covers I worked on, I couldn't get the end cap out in the end - it became stuck solid and I'm still contemplating how to get it out/whether it's worth taking out!

Someone on the rr.net forum (possibly leftlanetruckin?) said they drilled a wee hole in it and rattled a self-tapper down to get something to grip on.

Judging by how squished and brittle the old one was and how easily the cap came out compared to how much effort went to pushing it back in with a nice new O ring, I think I found my leak :-)

After a good ten minutes of running the bottom of the filter is still dry. Not nearly as horrible a job as some people seem to make out!

Good stuff.

The spare front cover I have with the seized plug has a 10mm hole I've drilled in it, but still won't budge.. one day I'll try and get a junior hacksaw blade in to try and cut a slice out of it to get it out.

I've brought this up, because I've just changed the oil and the filter was leaking as well.
I thought it came from up high where the valve covers were leaking but no, I've fired her up to make the level and I got a drop immediately rinning down the filter.

I had no time to look well in the region ... can it be the oil switch gasket leaking instead, or both? Which you recommend is an easier one to address?

It could be either but best to do both while in there. To change an oil pressure switch I found it easier to take the serpentine belt tensioner and it's mounting bracket off (on a GEMS anyway) so I could get in there from the front.

Yess, you must be right .... might as well do it at once. Never a rest moment!
However is the switch and valve relief work, I wonder if is only an o-ring problem then ...