Duncan h
http://www.thomashiggins.com/pages/collection-process-stageSome people use these and have good results.
Couple of things, you have to register with them, its free but abit of faffing about.
Online form to fill in and a driving license photo or passport.
Other thing is you need the customers name.
If you haven't and can't find it Last resort is land registry online think it's £3 gives the householders names this is only good if their mortgaged not renting.
Because house will be in landlords name not renters but the again if the landlord gets a letter from solicitor about money owed this could work as he would get the arse with his tenants.

The letter before action is good I've never got to that stage I drop the customer a line saying 14 days to pay if they don't then Thomas Higgins will be chasing the debt on my behalf and this will work out costly for them.
Sorry haven't got any letters been quite awhile since I've had a messer
Shouldn't have said that.
Good luck Duncan.