Shower valve delivering different temperatures

[craigjm]

One of the bathrooms has a three way shower valve, overhead, hand held and body jets. The hand held output has started to only deliver cold water but when I add a second outlet such as the overhead which is hot the hand held also gets hot. Then turn off the overhead and the handheld gets cold again. Water temperature to all other taps is as expected. Fed from a megaflow unvented cylinder

Faulty shower valve? what could be causing that?

I got another of the same type to prevent the need of the plumber coming out twice and I was surprised to see that the outlets on the valve are not the same size. It looks like its designed to take in more cold water than hot and the outlet to the overhead shower is fully open whereas the outlets to the hand held and the body jets are not fully open. Is this normal? seems odd to me but them im not a plumber

Should there also be inline filters on these?