Just bought a 316 had a few little hicups cam sensor etc but my temp gauge now isnt leaving the the blue hardly ......

Well, the gauge itself is still working, so hopefully its only the thermostat. Sounds like its probably sticking open. I'm going to guess you're not getting a lot of warmth coming from the heater either?

thermostat & sometimes coolant sensor if engine light is on with reduced cabin heat temp.

has this happened after the cam sensor change ?

Ye no engine light just mild air no hot air and the gauge i just on the edge of blue after a 20mile trip

thermostat your mostly likely culprit' <_<

Just to add...

Did you have hot air BEFORE the CPS change? If not then its either an open stat or air in system...

Other cause could include heater resistor (hedgehog)or even the heater control.

Ye had great air etc gt my new thermostat has arrived n im alryt to fit it myself etc no prob but just wondering If I needed to drain the coolant first etc to not get n air lock or something ....

I'm not much of a techie, Doug, but if you don't drain you're gonna get terrible messy, water peeing all over the place. Also, a lot easier to catch and refill your anti-freeze. Tip for when you refill your system, fill to top of expansion tank then run engine up with cap off. Will be a fair bit of gurgling and bubbling, keep filling as level goes down until everthing seems to have settled down. Any possible air locks have a better chance of clearing if not under pressure. I am told. Not being a techie. Probably.