Apparently it wants a modified crank case breather putting in. So does all the back cover ( where the oil filler is) come off without the need for the torx bolt out?
You'll have to forgive me if these are daft simple questions but I know not what I do.
My mechanic couldn't work out why it was there
The cover with the oil filler cap is the air filter cover - long 5mm allen key needed to remove it since there are two screws at the back that are a pain to get to.
The cover in front of that comes off with an 8mm socket I think - four bolts from memory, the crank case breather is under that front cover.
No need to take that torx bolt out to change the crank case breather.
Breather is under a plastic dome which is just in front of the air filter housing - more 5mm allen-head screws (including another one that is hard to spot).
You will need to unscrew the carrier for the fuel injector wiring (that is a small torx socket - sorry don't remember what size) and unplug the injectors - easy to do and they can only go back one way so no danger of getting it wrong.
It also improves access if you take of the fuel spill rail - a daisy chain of rubber/fabric pipes that connects the tops of the fuel injectors. Be careful with these, the plastic joints are notoriously fragile but not hard to remove from the injectors. Just move it out of the way and don't try to undo the pipes from the plastic T pieces.
Take off the breather cover, throw away the old filter which is probably a sticky black mess, fit new plastic oil separator (the modified breather) and put it all together again.
If you search this forum for crank case breather I am sure you will find pictures somewhere.