First thing Rob, you need to find out how much work they have got on. If it's only a couple of months but they're short of staff for the contracts they have, watch out. You may not be there for too long.
With the van I'm pretty sure that if it is only used for work then you don't pay tax i.e. you pick it up first thing on a morning and drop it off at night, using your own vehicle to and from your place of work. Your accountant should be able to tell you.
I was getting £15 a night food allowance 27 years ago and £25 when I was abroad, but you should be ok with that.
Not sure if I like the time off in lieu, that don't pay the bills, but if that's their policy, then it's something you'd have to go with.
Andy is right, when you break it down, £83 a day before tax, isn't going to make you a rich man, but if you're slow on work yourself then £83 looks a better bet than £0.
Regarding taking an apprentice on to train up, then you should be compensated for this, so I would negotiate on that matter. You are using your skills and passing them on and that is good value to the company in itself.
Again like Andy says, it could open doors for you, and with your knees, you might be stood up telling people what to do in a few months.
If you take it, then I would be looking at how they run the business as well as the staff they employ, you may be able to make improvements and who knows where that could lead to.
Either way, good luck Rob. Never regret the things you do, only the things you don't do!