Stuchbury - one thing to consider is the use of a pace band with both Km and miles. I make my own using Excel, print it onto card, cover it in Sellotape and fasten it around my wrist on race day.
All it needs is 3 columns - km, distance, mile. The distance numbers go from 1 to 42.2 (including 13.1, 21.1 and 26.2)
This way, you can check your times for miles and km and it is also a double check for your watch - as RS says, it will measure long so it will show you running faster than reality.
A footpod is basically a pedometer which links to your watch and will tell your watch that you are still running if you lose signal. Also usefull if you use a treadmill, so you can still use your watch, and for showing you your cadence.
The downside is, depending on which watch you own, they cost between £40 and £60