Brake light problem on a Ravenna

We have just bought a Ravenna on a Transit chassis and have being having a problem with the rear brake lights coming on even when the engine is off.

With no keys in the ignition you can touch the brake pedal and the light comes on. I presume this is a wiring fault and can't think of any reason why this should happen deliberately. Has anyone experienced similar or have any comments? At one point the lights stayed on and when the dogs are in the van I'm concerned about a flat battery,

I'm guessing we will have to get an auto electrician to look at it but I suspect that these fault tracing type jobs are expensive due to the time involved.

Brake lights working without ignition is fairly normal. Staying on is not.

I'm guessing this is a faulty switch. Easy to test, it's usually located somewhere near where the pedal pivots. On our van the brake lights are wired through the ignition switch, so off is off. However the horn push is live at all times, handy when someone is reversing towards you when you're parked up, but not such a good idea when a small grandchild discovers the button....

The brake light and rear side light are in the same bulb and weird things happen when the connections are crossed.