Thanks Pradeep, it had been set active but I disabled it when the rule failed. I'm just trying again having ticked the advanced box and made sure it's a before insert.

Still not working, I think I may have to define an inbound action that refers to the business rule but I don't know how to do that?

I had neglected to set the filter actions, seems to be working now.

Many thanks to all for their help

Take a look at the "Email Filters" Plugin - Email Filters - ServiceNow Wiki