Simon i need one Too !! the only place i have found that stock them (i bought replacement washers from them last time) is Gummers.
But Cheap it aint thats for sure !!
http://www.shower-spares.co.uk/sirrus/TS1500-TS1503_opac.htm
http://www.showerspares.com/products/spare_parts/sirrus_opac_ts1500ts1503_thermostatic
all the best
Matt
May 2008