So I'm playing at a wedding this Saturday, and I was practising last night in the church building, I noticed that when I was plugged into some power-points there was a bad buzz coming from my amp, which stopped when I touched the strings, so I guess it's a grounding issue...?
Then I tested out a few other power points and I found that I can keep the amp at the front of the building with a power lead from another power point in the foyer and there is no hum.

Could there be a problem with the buildings wiring? could their wiring not be grounded?!?! or is there something I have missed in my own set up?

I'm playing a strat into a valve amp, no pedals, low gain. It's not the hum of the single coils, and I tried turning off lights and moving around the room, only changing the power point changed the hum...

any suggestions?