Thermostat seem to be OK as temp comes up as it should and has never gone into the red.
Have ordered a new head gasket etc. As can't see what else could pressure the system as much.

So went to start my head gasket change and found (as usual) top hose to be hard and very hot but the bottom hose is stone cold!! I took the thermostat out and checked it in hot water and it opened at 89c, got and new one anyway and checked it before I put it in and it worked fine. Flushed the whole system and rad with hose no blockages. Went for a run to get up to temp and same again top hose hard and hot bottom of radiator still cold.
I suspect head gasket has gone, or has cracked, but why would this cause the coolant system not to work as it should???

Cos it can be a combustion gas leak into the system which whill block the flow through the rad due to back pressure, it's not 100% accurate on diesels cos it can be negative even if it's positive but if it's positive you have the answer if you use one of these: http://www.ebay.ie/itm/Block-test-k...594979?hash=item28067dbae3:g:BxcAAOxyjxlTMwY- , might be cheaper than a sniff test at a garage, though be aware that if you want to change the HG you need new head bolts too otherwise it's not a proper job IMO

Thanks, already got new bolts and head set.
Will take the head off and get it pressure tested to be sure.

That's the best thing to do