If I leave some flour and water mixed roughly and let it sit on the bench, after half an hour or so, its a bit bubbly and foamy, so I assume its fermented a bit?
Does anyone know whats happening here - is it better in the sun?
Essentially you've made a sourdough culture. Even though you're doing it just for batter, sun is probably too much for it and will likely have a drying, crusty effect . See the third link which includes the advice:
Does the temperature of my kitchen affect the starter activity?
Yes, very much. The starter will be most active at warm room temperatures, so if you keep your house cooler in the winter, it might be less active simply because of the temperature. You can move the starter to a slightly warmer location. The top of the refrigerator is good, or if you've got some other appliance that's a little bit warm, you can move it there. You don't want to get it too warm, so the top of the furnace or water heater might be overkill. Having it develop slowly isn't a bad thing if you're not in a hurry.