Please, please make sure that you give the torx head a tap before you turn it because the bolt head's been painted. I didn't and after getting three of them out, the forth sheared. That's when things got interesting/tragic........
Whilst fiddling with a pilot drill in preparation for using my thread extractor, I dropped it and it got stuck somewhere around the engine; but I couldn't find it.
I was about to grind off the head of the torx bolt with my Dremel (to help position a second pilot drill) and I was inspired to create a deep slot into which I inserted an engineers square and yes, it turned and I got it out.
I couldn't leave the drill I'd lost rattling around so I started to take off the panels around the tank in the hope of finding it. I removed the petrol filler which released the top plastic moulding but I wasn't expecting some rubbber bushes to be loose so one fell into the petrol tank. I won't repeat the langage used. So, the tank was drained and removed and low and behold, I found the drill bit. And after holding the tank above my head on the hour, every hour (it's quite heavy and a breathe tube makes access difficult) six hours later I got the piece of rubber out with the aid of surgical forceps.
The moral of this tale is: tap your torx!