Hi Malc,
Yes, you'll need to remove the thermostat, straight and curved plastic pipes to replace the four o-rings (CDU3858).
Whilst it's a bit more work, I'd recommend removing the plastic intake and front (left) inlet manifold. With them removed you have much better access to clean the mating faces in the engine block (with some Scotchbrite or similar) that the o-rings sit in and you can be confident that it's all properly seated. You will need a new manifold gasket (LKJ100992) though.
Hope this helps.