phase out collecting as much as possible.
over the next few months convert as many as possible to Standing Orders.
start with one weeks work and concentrate for the next few weeks in getting as many as is possible to go to standing orders,then move on to the next week and so on
start with a letter explaining you are going over to standing orders due to high cost of collecting,explain that if people want to be collected the price goes up to cover the cost,point out the benefits of the standing order so on and so on and chip away at it over the summer months so by winter you will see the benefits.
you will probably loose some customers or in my opinion,you will loose some bad debtors, don't be afraid to loose some,just replace with new and when you take on a new customer its on standing order only no ifs or buts set out a strategy,a plan and stick to it if it means a small reduction in your income now it will pay dividends in the long run and it will save you money
What is the percentage of debt compared to your turnover,£600 is small if you turn over £6000 a month but £600 is a lot if you turnover £1,500.
if you need a letter i am sure people on here can give you some pointers
good luck Bob