glush, I think you are correct on both points.
If you've already backtracked & lined-up 20m from the HP entry and are pointing the right way about to commence the take-off, then the Pitts shouldn't have entered in front of you. I would maybe look at it as you were taking off, the Pitts was taxiing, therefore you have right of way, Leicester being A/G radio.
Where possible I would always use as much runway length as I could, 20m may not be a lot but as you say, every metre can be crucial in the event of an emergency. A few years ago the club where I worked had a guy put a PA28 in the hedge at the end of the runway following an EFATO , I think an extra 20m would have made the difference that day.