I don't want this to turn into a heated debate when I was only asking for some advice.
I think some may have mis-read the scales/zeros.
1:50,000 scale OS Landranger Maps and a 1:25,000 OS Explorer Maps are both cheap. However, according to the surveyor who used to do our loft positions, both of these are not accurate enough. (Which is a shame because I own both of these maps). If my reckoning is right using a 1:25,000 map gives 4cm to 1km. If the pin hole is 1mm in diameter then this represents 25m. (Not too good at converting to old money, but I get this to be around 22 yards).
He recommended either a 1:2,500 or a 1:1,250. You can get these on line from the OS website. You get a sheet of A4, with the required post code at the centre, for about £30. I daren't even imagine the cost of a full size map!
Any how I am off to the library tomorrow to get a photo copy of their map. Never knew that a secretary got to find out such interesting things.