9a cooling issue

ive got a 20 16v 9a in my mk1 (its at a late stage in conversion/resto so has never been on the road) and I'm having a problemwith the radiator fan is not kicking in (radiator and fan are spurious..hella I think). I've tested the thermo switch on the radiator with a heat gun and it is switching and fan works when switch is bypassed so it seems the coolant is not circulating properly?. I've also swapped rings and a flange on the thermostat but no improvement. Any thoughts?
Thanks
Keith

first check over the system as per:
http://www.vwgolfmk2.co.uk/forum/viewtopic.php?f=37&t=14913

if you suspect rad is blocked feel it at various points after the thermostat has been open for a few minutes, you should feel the rad gradually start to warm up from the hose side across to the thermoswitch side. if it stays stone cold around the thermoswitch when temp gets to around 3/4 on gauge and your thermostat is definitely open (bottom hose hot) then this suggests rad blockage

Hella rads are good quality so no need to worry there