Prior to booking my MOT I just had a good look at my tyres and concluded that the front ones need replacing: I was a bit concerned that the tread seemed to be cracking on the outside of the driver side tyre so asked my wife for a second opinion and she pointed out that never mind the driver side the passenger tyre is showing the layer under the rubber.
@daved's suggestion of the Michelin Energy Saver+ doesn't seem to work, as far as I can tell from the Michelin website they don't do it in the same size as the current tyres (Michelin website recommends Primacy 3 or CrossClimate for the Volt). Maybe the Goodyear though they don't list my local tyre place as a stockists and I like to support the village shops.
Edit: apparently they can get the exact same Michelin Energy Saver tyres as current ones so I'll be heading back this afternoon to have them fitted.
Oh, and in case you're wondering: