Company charging interest

I have a client that sells security systems for domestic users.

The majority of these are sold as cash on delivery/payment in advance. Due to the current climate, more and more customers are paying by Standing Order over 36 months, and our client is not charging interest on this.

I have advised them that they are losing out on the interest themselves, and taking a risk, and therefore need to consider charging interest.

I am wondering if anyone knows which specific areas of legislation, i.e. consumer credit act, need to be considered and if there is a website I can visit that will provide more information on what to do etc..

Any help on this would be greatly appreciated

Thanks

David Bowler