6% is 6% Wayne, that's a cracking result for any direct mail campaign. Normally it's fractions!
I wish I could get 3 in 50 of my old customers to book a job when I mailshot my list.
The number of letters you've sent is tiny for a mailshot... obviously the less time you've been going the less old customers you'll have who are ready for another clean, so the actual result in ££s won't be spectacular.
I would take this as a very encouraging sign that you're doing something right. Just think in 5 years time when you send out 1,000 mailshots, if you get 6% of that you'll be more worried about tax avoidance