Ah I didn't realise they had a flow restrictor, I might look into getting one. I just have a standard Y fitting currently. It's got clear hose on it though and I can see good flow at idle so it's definitely moving water through the core.
The fast that it only gets hot under load leads me to believe it might be a sticky thermostat. If its stuck slightly open its never going to get to its full operating temp in 50-55 degree water (it's still winter here).
Will see what I can find.