Unfair dismissal:
http://www.direct.gov.uk/en/Employment/RedundancyAndLeavingYourJob/DG_10026692Fair reasons for dismissal:
http://www.direct.gov.uk/en/Employment/RedundancyAndLeavingYourJob/Dismissal/DG_175834Conveniently I've been learning about dimissal laws at college this week.

You said your boss has to look up the laws of dismissal, it sounds as if he's going to fabricate a reason for dismissal to coincide with the laws.
You've had one warning over a year ago, you've made this one honest mistake over a year after your warning, there's no way he can sack you fairly. If he does give you the sack, fight it and you'll win.
As for gross misconduct, from what I know that usually covers putting people in danger fighting etc, so you should be ok for gluing the wrong side of a piece of plastic.

Alternatively you can take simonpolly's advice and let some arrogant pig sack you unfairly and leave you jobless in this economic climate where there are no jobs and join the other 2.5 million unemployed people that there are already in the UK.

I was only joking,no way would i pay him back it sounds like it was a honest mistake, if he sacks you just take it on the chin and move on.
First post yay!