4.2 S/C - I've just replaced a sticking offside rear caliper and now all of my disks are feeling hot after a decent drive. Any ideas? Brake fluid needs changing? Something wrong with ABS?

How hot? Fronts hotter than rears or vice-versa? Heat is a product of braking so 'normal' up to a point, if the fronts are roughly equal temperature side-to-side and warmer than the rears, also roughly equal temperature side-to-side, then it sounds like normal behaviour.
Is there any blueing on the discs, any brake noise or grabbing after driving for some time in traffic, does the vehicle pull to one side under braking or shudder / vibrate through the steering?

Disks should feel hot after a good drive. If the wheels feel hot as well you have a problem. 2012 TDV8 Vogue (L322)

All wheels are cold....front discs are cooler than the rears which are both so hot I can barely touch them - but both sides are about the same temperature so it could be normal? I am noticing some vibration coming from the front offside when I first pull away which feels like the brake sticking - which is making me think the issue is something effecting all of the brakes

Jack each wheel and see if there is any binding 2012 TDV8 Vogue (L322)