start from cold, sit with the engine running high idle and check the water pipes to the radiator - if they warm up gradual there is an issue - should be cold until engine is up to temp and then stat opens and hot water is released to be cooledthere may be a little leakage of hot water - but should not be a lot and you should notice a distinct increase in temp.
should readily available from most good motor factors
55037 539AA
cant you just take the stat out a check its closed when cold and open when dropped in boiling water?