Thanks for the informative replies gents.
Pilot DAR, that's a very good idea with taking photos of the relevent pages. I have a 12MP D-SLR so I guess that a full resolution photo on that with a bit of cropping/editing I should then be able to turn that in to a fairly good copy.
SkyHawk, I will try the idea above next time I'm at the airfield and if not give your link ago to just purchase a real one!
In the meantime I'm currently trying to plan a trip for this weekend - would you say that aslong as my 172P is no more than 2300lbs and the take-off/landing performances for the 172N work with a lot of room to spare (say >20% once all the other factors of grass, headwind, 1.33 x safety factor are worked out) that I should be ok? I guess that if my margin is less than 20% then the figures are a bit too close for comfort, and I could do with driving up to the airfield and sourcing the actual POH just to be sure.