The DTI's website is a good place to start:
http://www.dti.gov.uk/renewables/renew_1.3.4.htm
That site refers you to the Clear Skies programme for information about grants. This is where things get a little confusing because the Clear Skies site says that the main grant scheme closed on 3rd March. However it also says "With the exception of applications for grants for solar . . ." which suggests that grants for installing solar panels might still be available. If so, the grant is a flat-rate figure (irrespective of the size of the installation) of �400.
I suggest reading through the Clear Skies site and contacting the people behind it for the latest information:
http://www.clear-skies.org/
Chris