Indexes and some registers are available at the:

Society of Genealogists
14 Charterhouse Buildings
Goswell Road
London, EC1M 7BA
Telephone number: +44 (0) 20 7251 8799

Write to:

H M Greffier
Royal Court House
St Peter Port
Guernsey, GY1 2PB
Channel Islands
Telephone number: +44 (0) 14 8172 5277

You can also try the GENUKI Guernsey website for further information.