Cozza spotted it!
Yes indeed,all the metalwork is grounded to chassis to prevent hum pickup,but due to the double insulation system used(virtual ground?)non of the equipment is actually grounded to earth.
A standing voltage can develop between the equipment and earth,sometimes as much as 50v,quite enough to cause a tingle when touched.
If a signal distribution system is connected for radio/tv signals,this can make things worse,and can cause odd problems with remote control operation.
Sky+ boxes are rather prone to this
It's probably common in display set up's,as they use multiple outlets,and of course have synthetic carpeting/dry air etc which exascerbates the effect.
Nick