Vauxhall Agila temp sensor and coolant leak

The problem with my car is that it is struggling to start all the time now and when it does my temp gauge hits the roof, i took the temp gauge out and tested it in my nice cold pint of coke and it was registering roughly bought 90 degrees which made me think it was just faulty. Checked the oil and the cap has a mis colour in it like bailies but dont believe the head gasket has gone, looked it up and said most likley a service. Also got a very small coolant lek which i cant even find but still got pressure in the pipe. What i want to know is...

Could the temp gauge be playing up with my engine managment in thinking that the engine is hot therefor causing my car to strggle to start first thing in the morning and even if it does start it keeps dying on me until i drive it, also pulls going from first to second but got no loss of power after that.

any ideas or people with similar problems would be greatly appreciated before i take it to the garage cause i aint getting scammed, dont trust garages so try and figure out most of it before i go so i can tell them exactly what is wrong with it.