Concensus on the oil leak seems to be that they pretty much all do that anyway and I should just keep an eye on it until I either replace the head gasket or it gets worse – I can live with that 
This weekend I spent most of Sunday mixing filler, applying it really carefully, sanding back down (with 80 then 180 grit paper) then giving it a squirt of colour and lightly sanding again to look for low spots. After three rounds of doing this on the front wing where I’ve got rid of the bumper mounting holes, I still had two small depressions in the surface that had been there when I’d started in the morning 
I decided to give it a rest, as my “filler mojo” was obviously somewhere else 
Turned my attention to the tailgate and the last problem area that’s stopping me from using the car during the week – the leaks around the rear screen. There’s a reason we’ve got so many reservoirs ’round here and it ain’t cos’ of all the sunshine 
I’m starting to think that the only thing in the previous owner’s “repair kit” was a tube of bathroom sealant…
Spent a frustrating time working out how to get the screen out by folding the rubber seal in on itself like the vague instructions in the Haynes, until my girlfriend came out with a cup of tea and asked if it wouldn’t just be easier if I pushed the seal in from the inside if I was going to be pushing the screen out ? 
Minutes later the screen was ready to pop out 
With the screen out, I can see that most of the rust is along the bottom lip of the screen frame, which should just be a case of chopping out the rot and blending in some fresh metal – the skin’s pretty solid with just a little surface rust to clear.
There’s not alot of compound curves to worry about, so it should be a good job to get underway with. The tailgate’s now off the car and awaiting clean up.
I know that the easy solution is to just pick up a less rusty one and repaint it, but I think that goes against the grain of a budget resto when I’ve (theoretically) got all the bits to do the repairs myself – given enough time. Besides, tailgates with no spoiler mounting holes probably attract a price premium nowadays 