Take a look at:
Kent GLiding Club.
The comments below about it not being the best "soaring site" are a little unfair - I regularly get 4-5 hour flights. Where it does lose out to, for example, places like
Shenington is for cross country flying.
Where it wins out is the facilities and equipment.
Looking at your location and the types you fly I was wondering if you fly with the Tiger Club. Some of the check pilots there are also instructors at Challock - I'm sure they could answer all of your Qs.
OC619