the inlet manifold is a **** because if the bracket underneath. 5 hours and it still wasn't off. I am going to get the longest extention bar the shops sell and try again at the weekend.
I take mine off from above but mine is a P1.
Procedure is this.
1. Remove inlet trunking up to the throttle and plastic inner wing cover.
2. Unscrew the expansion tank ( 2 screws on top, 1 13mm bolt underneath ). Just slacken the 13mm, no need to undo it as the tank pulls up off the bolt. Swing the expansion tank out of the way towards the radiator. No need to drain the coolant/remove hoses.
3. If you have a bracket for the heater pipes and wiring under the throttle body, remove it ( 3 x T30 Torx on a P1 ).
As long as you have reasonably thin arms, you should now be able to undo the 2x 15mm ( centre ) and 2x 13mm ( ends ) bolts by reaching under the plenum. Still not easy, but I can get my plenum out in about 20 minutes tops ( including throttle cable, wiring etc ).
If you have a P1, the belt side 15mm bolt is impossible to remove with a ratchet handle owing to a steel tab on the support bracket. You'll need a very stubby 15mm spanner.
Whether the procedure works on a P2/3, I've no idea, but it should be similar.
Edit : Obviously, you'll be working by feel for all but the gearbox end bolt. And move the expansion tank back into position asap to avoid stressing it's hose for too long.