I have a Classic Mini Cooper but unlike most cars, there is nowhere to plug in my Sat Nav so when it runs out of battery I can't use it. Does anyone know how to get around this? Is there something I can buy which will let me plug in and recharge my Sat Nav in my car?

You need to wire in a power outlet.
They are freely available.
Here's one from Halfords: Halfords 12v Hard-Wire Cigarette Lighter Unit from Halfords Price £7.99

Depending on the age of the car really depends on where you can site this.
Slightly harder with a full length dash, but could always be hidden away.