Dave Croft wrote:

I suspect somewhere around 2.0 to 2.2 volt but I cannot find
anyone selling bulbs like this in the UK.

And you never will.

Hook six of these batteries in series to get 7.2V. Over-voltage a 6V
MR16 bulb by 20%.

See "http://www.e-go.gb.com/parts.asp" to obtain a 6 volt/5 watt MR16
lamp.

I haven't seen MR16 6 volt lamps, at more than 5 watts, except
integrated into some emergency lights, and these don't have a front
glass attached, see
"http://www.cooperlighting.com/specfiles/productinfopdf/EADX0497.pdf".
Steve
http://bicyclelighting.com