Gareth,
Being as strong as you are with customizations, I am surprised you have not looked at creating a custom Workflow Rule to do this. At the same time, you may want to consider our Audit Trail and Stay Tuned apps from the Apps Marketplace which will monitor changes to work items. Stay Tuned even creates a thread named Recent Activity and logs changes there for you to see. Here are the links to the Apps...
Stay Tuned -http://www.clarizen.com/appsmarketplace/item/stay-tuned.html
Audit Trail -http://www.clarizen.com/appsmarketplace/item/Audit-Trail-For-Work-Items.html