Right then, went to Towcester to go shopping, about 10 miles away, all dual carriageway, kept it at constant 70mph. Bear in mind I currently have the drilled stat in. Stayed below 90 degrees most of the way, couple of times did go up to 92ish. On arrival at Tescos, I had lost probably 1l of coolant, nicely sprayed around suspension strut and airbox.
So we did our shopping, and I then filled the coolant back up, and put on the cap that Omegatoy got from the diesel in the scrappy. Drove back, constant 70mph, temperatures same as before, mainly near the end stop, but occasionally rising up to 92ish. However, upon arriving home, the cap was hissing (this cap even hisses when you drain coolant, but forget to remove cap, so I had written it off as knackered), but had lost minimal coolant. Undoing the cap slightly, and it did exactly what I would expect a hot car to do - hiss voilently, and start to boil in the bottle. Radiator was hot all over - too hot to hold for too long.
Feeling like a better test was needed, I wired up the leccy fans (and the intercooler fan) and took it for a decent thrashing, only to be prevented from doing so by today's ridiculously slow drivers. Opened it up when I could. Temp was around 90 - 92, dropping back down to endstop whislt following grandma and grandad. Again hissing, but minimal coolant loss (though defo still some)....