The top two cross runners on the radiator have swollen up and one has sprung a leak.
Has it just gone weak with age or is it a sign of head gasket failure?

I did find some oily sludge in the header tank which I cleaned out last weekend, but it doesn't seem to have returned. I also put Rad Weld in the radiator to stop the leak - that lasted about one heat cycle before it started leaking again
There's no water in the oil. I was wondering if cylinder pressure could pump up the rad with a blown HG?

 

Check to make sure you cap is actually relieving pressure as 18psi.

 

That's an interesting point, I hadn't thought of that. I replaced the factory rad cap a few months ago as the seal had failed. I got a replacement metal cap from Summit rated at 18psi but I guess it could be a little outside of that range.