Here you go:
27.2 miles with 940 metres of ascent. Start from Mere.
Head north across A303 on minor road leading to b/w then turn L at t-junc for White Sheet Hill. Exit car park and straight over on b/w towards Kilmington Common. Straight across onto minor road that leads to Alfred's Tower.
Head NE through King's Wood Warren until minor road, turn R then L onto b/w eventually coming out on minor road at Gare HIll. Turn R onto b/w heading NE to B3092.
Straight across onto minor road into Horningsham then take b/W on R in a S direction then E. At minor road turn L and R onto b/W heading S through Bidcombe Hill/Whitecliff Down.
Head SW through Bushcombe Bottom leading to Kingston Deverill B3095. Turn L towards Monkton Deverill then R onto b/w heading S to meet A303. Turn R for short distance then R on byway to Mere down/B3095. Straight over, this heads to White Sheet Hill where you were earlier.
Hope this make sense but can be more specific, let me know.
If anyone wants a gpx, let me know.