It's certainly the kind of Big Project I'm thinking of. You'd need to roll out sell-back junction boxes (they're commonplace in greenie-land now, Good Energy do 'em as an option but I have no roof nor open land). The Grid may need infrastructural work too, dunno how that would go.

Here's a back-of-envelope calculation that suggests that if your scheme went ahead, it would score 100% of daytime summer needs, 20% in winter.

http://lightbucket.wordpress.com/2009/03/09/a-second-look-at-solar-power-on-roofspace/