How can I find my wheel size I was wondering if anyone could help me find out what size rear wheel I need to buy as a replacement (wheel is EXTREMELY out of true) as I cant seem to find it.

Bike: REACTION HYBRID HPA PRO 400
Tyres currently on bike: 28 x 1.65 - 700 x 42c

Thanks in advance,
Dan
This is actually a mountain bike, despite the name (hybrid referring to having a battery, it seems). 

You need a 700c (29", ISO 622) rim with a 135 mm OLD hub (a mountain hub spacing) and mounting for discs (your existing rotors are centerlock, so unless you want to use a different rotor or adapter, you should get a hub with that). Width compatibility is another thing to look for -- very wide tire on very narrow rim and vice versa are not good combinations. The freehub should be compatible with your cassette. 

Note that a wheel that looks extremely out of true can often be made to work well with a bit of adjustment -- unless you're sure the wheel is a gonner, I'd try that first.