Overheating still - but no "mayonnaise"

I've had overheating problems on and off for months, and I now think it has to be the head gasket, BUT I can't see oil in the coolant and I can't see any emulsification in the oil. However, can't really think what else it can be...

Borrowed the village garage's compression tester (for the radiator, not the cylinder sort!) on Saturday morning and found a major leak where the aftermarket section with the temp sender in was not quite seated properly. I'd checked that one before so I think it's been working itself off slowly. We also bled the radiator whilst we were at it. I then drove a mile or so to get any more air around and then bled it again about two hours later before I took it out properly. It only took about another cupful of water that second time so I'm convinced it's not an airlock.

Took it out and couldn't persuade the temp needle to even twitch from running temp for around 30 miles. Then I saw DaveW on the M60 and managed to warm it up good and proper. :D

I backed off when the temp gauge started to climb and went off a couple of junctions later to let it cool - temp gauge got to about 3/4 and did start dropping whilst I was idling. I had a look for steam and found another very slight leak which I fixed by tightening the jubilee clip.

Drove back across Manc to the Trafford Centre and left it for half an hour for shopping and a coffee. Came back, idled for a minute or so, and pulled off. Soon as I got into 3rd gear away the temp gauge went and I managed to find somewhere to stop just before it hit the top mark - the rad cap was letting off just a little steam at this point.

Called the RAC knowing I could get a compression test done again that way - the guy was very helpful in general, in fact. We topped it back up and then checked it was holding a bar for a good couple of minutes - it certainly did. Rad cap tested OK too.

Turns out the low-loader company they use had a logistics centre about 1/2 a mile from where I'd ditched so we drove that carefully once the car was cool. From the evidence of that short trip I think the head gasket's dead.

The car had done just over 60k when I bought it so C&S have already changed the water pump and timing belt and they also discovered the viscous fan was stuffed. So, all those usual things are new and confirmed working (and there was indeed evidence of weeping on the old water pump.) A/C belt was squeaking when cold so he changed that too.

Thermostat is definitely good - the return pipe is red hot and the bottom pipe is nice and cool, which also shows the radiator must be working to some capacity.

It's now reaching high pressure VERY quickly on idle so I think that's it, goodnight Vienna...

My theory is it's been a dodgy gasket all along and the leaks venting the system artifically (and the dodgy pump/fan originally) have just disguised the problem. Does this sound reasonable?

Anyone any other ideas? Really don't see what else it can be now.

Zack