The Competitions at Stampex are held under the auspices of the Association of British Philatelic Societies. They are organised by The British Philatelic Trust who will be pleased to distribute rules and directions to collectors wishing to enter.
The contact details for the Trust are:

The British Philatelic Trust
Room 101
Business Design Centre
52 Upper Street
Islington
London
N1 0QH

Telephone: 0207 688 8433

Alternatively entry forms may be found on the ABPS web-site www.abps.org.uk