I couldn't find anything on the RHS site but this is the Bucks
organiser for the National Garden Scheme. She must be able to help
you: County Organiser:

Mrs Maggie Bateson,
Fressingwood,
Little Kingshill HP16 0EF
Tel & Fax: 01494 866265
Email:

and this link or someone on it might be helpful, too:
http://www.gardenersclub.co.uk/societiesinfo.asp
--
Sacha
www.hillhousenursery.co.uk
South Devon