Chemspec do a product called ultra fresh that can be spray applicated. It is not listed as a pesticide because they did'nt want the cost of listing it. Does same job though and is about £3 for 100ml (dilutes 100-1). As far as I remember it does'nt have any permetherin either.
You need to be liberal with application and make sure you get into and under/behind everything. Clarify to custy that deals with existing problem and not necessarily cause. If they have lots of pets etc.