to check for a blown head gasket you can pressure test. if one of the cylinders is low then you have a leak! as regards to the temp have you tied a new sender? they are not expensive for the CVH also if money's tight get one at a scrapper for a one or 2 quid! another trick to try is vacuum filling your coolant system. This worked really well for me NO air locks at all . If you are completely screwed pm me!

I find it helps a lot to use the vacuum method as I have modified my cooling system!

I have also used "Red Line Water Wetter" this is really good too as it makes the water better at moving the heat out of the engine!