Woohoo, new stat fitted. Doddle if you know how. Those 5 hours last weekend getting it apart to find the local motorfactors had sold me the wrong one, really helped.
Today, drained, apart, and back together in an hour and a half, and another 40mins to fill and bleed (been colder than a witches tit here, so took a while to get upto fan cycle temps).
Made sure I opened and closed the heater a few times too. that way I'm not going to get air trapped in there too.
Only took roughly 4 litres though again (like last time). I'm guessing, as it was a drain and fill, and not a full engine stripdown and dry build out of the car, that some of the coolant will stay in the block somewhere (like when you change the oil it never takes quite the expected amount).
gave it chance to cycle the fan 3 times, then switched off, put the cap on, and let it cool for an hour. Checked level, then went for a nece blast out the M62, did my usual route to work and back to Salford, and then did some crawling in traffic. Gauge got to dead middle and stayed there. Fan cycled more often too.
Only thing was, now it is staying at temp, it mean't the old dumpvalve hunt/stall is back.
Played with the dump valve tension (I love adjustability), and the throttle speed screw (yes on mine is does make a difference), and now it will idle ok too, although the dumpvalve locknut and the 10mm bolt on the alternator snorkel clamp had both managed to disappear during my test drive, and the snorkel was tryign to make a break for freedom.
Not the worst job I've done on a car (that would be removing rusted on rear shocls on a Skoda Favorit), but certainly one of the fiddliest.
Lets see if I can improve on 22MPG now that it won't think it is running cold 70% of the time.