Ecotricity have been recommend by Which? magazine.
Ecotricity is one a three suppliers not subject to price cap as investing in new renewable supply called Ecotricity. The company founder and owner is Dale Vince.
They are investing in allsorts of sustainable enviromental friendly energy production, including biomethane made from grass.
They do a referral offer. The old links for this used to be have a diffrent code (remove web archive part of url prior to second https:// to use) which resulted in £25 per fuel credit when you sign up with them, but my code no longer works and they seem to often have better sign up offers on their website anyway. See some of new links below. If you use Facebook then there is an unused Ecotricity unoffical forum group.
I will also added related links including Biomethane below: