FFF, you will probably find that the CAA charts are to be used for public transport flights ie fare paying passengers as I would guess that your aircraft will be public category if it is a school aircraft. The use of 25 degrees of flap on PA28's is a short field technique. The CAA tend not to allow short field techniques when an aircraft is engaged in public transport work (it was the case with a Seneca that I flew). There is nothing wrong with using the manufacturers figures for a private flight and using the recommended factors, though it is recognised that these can be a little optimistic due to marketing considerations, wear and tear etc, hence the CAA recommendations. If a 4 up warrior cannot get into the air safely at Compton Abbas then we really are missing something!! Enjoy your day out, long range forecast looks promising!!