Shaun
The number of years you've been doing it, I can only think of a few reasons why anyone would continue to advertise and some don't apply to you anyway.
To grow the business i.e. another van.
Upgrade customers (ditch the bottom 20% and replace them)
New lucrative market ie commercial, super high end clients
Moving to a new area.
You've got a quality database, you have referral systems in place.
You are busy even with your 427 so why advertise unless you want something different, or in a really different league (hourly rate wise) cos you are still swapping time for money, in CC at least.
I scratch my head when I read about one man bands dishing out 20k of leaflets month after month 20 years down the line. Either they have a bucket with a hole in it or they have very poor referral and repeat systems in place OR they are looking to expand beyond one van.
Mike