Fault with Gainsborough Shower Mixer?

We have a gravity fed gainsborough shower which is about 10 years old. If someone has a shower and another then goes in to it a few minutes after it has bee switched off the water is burning hot. You have to wait for a while before it returns to normal.\r\n
In the mixer there is a small screw that you can remove and then insert a small screwdriver and locate another screw and turn it (can't remember which way to turn up/down temperature). This can fix it temporarily. However, you have to do the same again next time 2 people use the shower.

Can I fix it or can a plumber fix it?