Thelia 23 hot water problems?

Just moved into a property with a thelia 23 heater and I'm having problems getting the water hot enough for a shower or a bath! When I run the hot water the pilot is igniting and the water does warm up slightly however I'm still left with quite a chilly shower! The little knob which controls the priority of hot water is up full but still this doesn't help. The pressure is at about 1.5.

Apologies if this question is a bit stupid but this is my first house and as I'm sure you can all understand my Missus is giving me a very shed time about it!