With a couple of runs sufficiently further than just up to town in prospect to give things a fair trial I let the coolant level in the expansion tank down low enough to be sure the UV dye would be well mixed after topping up.

After 6 cold starts and 150 miles on the clock, the coolant level hasn't visibly moved. UV torch reveals bright violet in a nice strong Jackson Pollock pattern all over front and much of the top of the engine. Light dusting on thermostat housing, sides of radiator, inner wings and exhaust heat shield with pretty complete coverage. Inside of radiator mostly covered to more than dusting but less than Pollock standards.

I'm guessing a pinhole leak or three on front side of the radiator being picked up by the fan and thrown everywhere. If the team agrees whats currently a decent price / performance radiator for a year 2000 HSE 4 litre. Price range for acceptable breeds seems to be around £100 (Island 4 x 4) to £170 (Euro Car Parts). Can collect from Euro Car Parts so a few pounds extra is OK for security of it not going via common courier and also having somewhere to take it back to if it does turn out bad.

Guess perhaps I should do the thermostat at the same time. Allmakes, Bearmach et al being around £20 which is neither here nor there. Hoses are all new reasonably recently so should be OK. Not done the little one off the top of the radiator but no apparent leaks from that. Heater O rings still good. Do I risk disturbing them so I know its done or let sleeping dragons lie.

Do I have to get the £15 bottle of special spray from Ring to shift the fluorescent residue or is their something easier to find that works?

Clive

I use this Clive
http://www.ebay.co.uk/itm/Interdynamics-Clean-Away-UV-Ultra-Violet-Dye-Cleaner-Leak-Detection-237ml/121342604288?
As for thermostats- had mixed results with Bearmach recently. Compared back to back around 7 degree variance in opening/ closing temps IIRC. No jiggle pin in them either.
Gates are the only ones (admittedly from a sample of 1) that actually opened at 85 and had a jiggle pin.
Heater O rings- if you've got a borescope then give them a good once over. If no leaks, leave them until they do :)
I'm guessing you don't have a static cooling system pressure test rig? That's the best way, with coolant warm and in conjunction with dye, to pinpoint leaks. No running engine removes the flinging effect.

Thanks for that.

Ordered the UV dye cleaner from your link.

Will a pump and an "operated on" pressure cap be good enough for a pressure tester. Old pressure cap lying around somewhere. Pump and gauge I made up 20 - 30 odd years back for something else. Finding it may be challenge!

Clive

Only needs a small pump (mine's like a shiny bike pump!) as blow off pressure for the system is 1.0 bar (15lbf/in2). I'm sure that with a bit of creative engineering on the cap you could mod it to make something.
I've found that if you take system pressure to the max on a hot engine, you can induce weeping even on perfectly good O rings and hose joints, so I test to 13psi myself!