So here is a possible solution
This is my iPhone, a Belkin headphone extension and a spare front rubber grip for my transmitter. Here's a better picture of the extension cable. It's designed so you can use normal headphones on your phone, it has a built in mic but more importantly volume controls.
I have fitted the volume control into my spare rubber grip.
The clever bit is an app called Eyefree stopwatch that allows you to use the volume controls to start, stop and lap the stopwatch..
At the moment it's just held in with tape need to work on the location of the switch and give it a test. If it works out I'll find a better way of fixing it all together