I don't need any cans of spinach to shine some low-energy light on your dilemma. You're spot on: the way the electricity market currently works makes it hard to judge whether consumer demand makes any difference at all. Each unit of renewable energy generated is awarded a certificate which can be sold from company A to company B. Company B can then use the certificate to count towards its legal obligation of supplying a certain quota of renewable electricity. So instead of two units of renewable energy existing, there is only one that is accounted for twice. In other words, fossil fuel-burning companies can simply buy certificates to cover their renewables obligations.
The kind of society-wide shift to a new power base favoured by Red Pepper is more likely to result from supporting electricity suppliers that source renewable energy and hang on to the certificates. This would limit the circulation of certificates, and encourage development in the renewables market. See Friends of the Earth\'s guide to green electricity tariffs for more details.
But consumer demand for renewable electricity sources must also increase massively if it is to influence the development of the green electricity market in any useful way. Your change of electricity supplier may indeed fizzle and fade as an act on its own, but if all your mates who bang on about wars over oil similarly stopped guzzling the stuff, you might spark something interesting.