How can I locate my main external Stop Tap?

Three Valleys Water supply my property with water. I need to move my home's internal Stop Tap and so to do this will need to turn off the supply using the external Stop Tap.

I have been only been in my new home a few months and haven't found it. I fear it might be under the driveway installed by the previous owner, or buried in the front garden.

Are they any plans which might show where this is? Are there any gadgets I can use to help me find it?