Have a look at [broken link removed] this will give you a good overview of the requirements of a good system. I would say it's less about being green and more about making your system as controllable as possible to save money, without the correct controls you are wasting money. As a minimum i would want timed control for heating and hot water, with a room stat(s) controlling the temperature in the house and a clinderstat controlling the cylinder and a good flush/power flush to make sure everything stays clean, for a wish list i would have TRV's and a boiler with a outdoor temperature sensor which can reduce gas usage dramatically (in some cases). I can't answer your question about the grant but for which boiler, i like Viessmann and Worcester Bosch, it's always good to find out about the service network in your area before you buy a boiler(ring the manufactures and ask) because there is no point having the best if there is no back up, which seems to be the case with Valiant(shame,very good boilers) you can buy cheap but there is a big difference in the build quality and controllability between a high end boiler and a cheap one, get a few quotes which is always the best way to get a feel for a price, Gary.