(1) The CAA guidelines are summarised in GA Safety Leaflet 7b, but the definitive stuff is in AIC Pink 12/1996 pink 120. However, it gives no advice about the effect of headwinds, since the CAA view (conservative but sensible) is always to use the still air figure when taking off into wind.
(2) More usefully is to find somebody who has a flight manual for an old BCAR Section K aeroplane, with similar performance and adapt the WAT chart (scale geometrically not arithmetically). This is a rather cunning graphical way of factoring slope, wind, temperature, etc. to come up with an accurate take-off distance figure.
(3) However, my gut feeling is that the only sensible approach with this sort of slope is to always take-off downhill, and land uphill, particularly with trees at the top of the hill. Landings should be straightforward, and take-offs if a bit marginal presumably you're over descending ground so long as you're airborne. What is vital in this case, is to be mentally prepared for an early go-around decision during approach !
If the available runway width is sufficient, you could do worse than leaving the grass longer on one side than the other, and land on that when the grass is dry.
G