Just building on what Whirly says
An ex-CSE Warrior (I fly one too) like the one at Sherburn will have an empty weight of about 1550 lbf, and an MTOW of 2325lbf, so that's a useable load of 775 lbf. I don't think well in pounds, so let's make that 352kg.
Assuming an average 13 stone adult (in clothing remember) that's 83kg x 4 = 332kg.
352kg - 332kg = 20 kg, or 28 litres of fuel.
That's roughly one hour, with no reserves and no toothbrush.
Sorry I didn't point that out earlier. Best replan now before your CFI publically does it for you. I'd suggest leaving somebody behind is the ONLY way to do this, since:-
352kg - (3 x 82kg) = 106kg.
- 4 hours fuel carefully leaned (28 x 4 x 0.72 = 80kg) gives 26kg left.
Which is just about enough for some light bags (57lbf, just slightly more than Whirly's best-guess).
G