Where to get gear shifter for 31.8mm handle bars? I'm based in the UK and have bought some new handlebars that are 31.8mm for a hybrid 8 gear bike.

My old handlebars were smaller in diameter so my old gear shifter won't fit. No gear shifters online actually say the diameter of the handlebars they're made for. Is there a standard diameter for mountain bikes and road bikes? Can someone link me to an example 8 gear shifter that will fit 31.8mm handlebar?

Any help really appreciated.
22.2 mm (7⁄8 in) is the standard diameter for the brake, shifter and grip area of MTB handlebars. 

I think you bought a bar that was designed for bar-end shifters only. Easiest way out of this is to swap the bar for one that allows shifters and brakes to be mounted on the area between the stem and forward turn.