We bought a "Fresh-up Set" last year so that we always have a spare cassette in the boot. From Jacksons Leisure on the Wirral:
Excellent service too.Thetford caravan and motorhome cassette toilets C200 CW CS C-250CWE Porta Potti
EXPRESS DELIVERY ON Thetford and Dometic Chemical Cassette Toilets for caravans motorhomes and marinewww.jacksons-camping.co.uk
Thetford do a special box that will fix to the floor or underneath your van to put your spare cassette in to keep it secure - important when your carrying a full one