Well, just thought I would update to say thanks for all the advice and tips. Pleased to say that (touch wood) the boys are now doing very well and we've had no accidents in the house for a few weeks, and they seem to have lost interest in eating poop (thank goodness!)
I think what made the most difference was changing their food to JUST dry stuff, and reducing the amount they were eating each day slightly for a while. They seem to have it sorted now though, at just over 4-and-a-half months old they haven't done too badly!
I think I'm tempting fate by posting this as they'll probably have a relapse tonight now (!) but I wanted to let you know that all your advice was extremely helpful! Thank you :)