Locating 3 way CH valve

[Jon89]

Hiya. We recently moved into our home and have a problem with the central heating and hot water system. The boiler is a glow worm and is about 10-15 years old at a guess. The hot water works but when the boiler is on for the water the heating comes on as well. A Gas engineer has said that the fault will be the 3 way valve but we can't locate it. The previous owners had the hot water tank moved into the loft but the plumber who did it says that the valve was not by the old tank. Does anyone know the best way to find it please? Thanks in advance

 

[Ben-gee]

To be fair I think I can see the tundish so that’s ok. It’s just that now the cylinder needs to be drained to work on the combination valve.