Running too cool/Useless heater?

Hi All

I used the wife's 05 Cooper the other day. After a few miles I noticed the heater was on full but the car didnt feel that warm and the hot air was more warm than hot? Temp gauge is barely reaching the 1/4 mark.

So at 95k it may be new thermostat time unless you can think or know of any other thing that could cause it?

Anyone fitted anew thermostat to one of these? Easy job or nightmare? Any advice regarding changing one?

Many thanks