I have seen claims that you can cut energy wastage by over 75% (about £7 a year for an average energy bill or about £65 over the bulb's lifetime) by using Compact Fluorescent Lightbulbs. Of course, this depends how much you spent on the original bulb. Sixty-five pounds might be over-ambitious if you paid £35 for the bulb in the first place.
This website has a graph showing you how much money you save by using CFLs.
The Energy Saving Trust is an organisation that it looks like I'll be referring to a great deal. This is the lighting specific link: http://www.est.org.uk/myhome/efficientproducts/lighting/
Here are some offers for energy saving lightbulbs:
Powergen - They will give four free energy saving lightbulbs to people on any qualifying benefits
Also, some councils offer to swap your normal lightbulbs for energy saving ones, whilst others will give away lightbulbs to certain groups of people and still others will sell them to you at a reduced rate. It could be worth calling up your council.
Whilst looking into this further, I did find other sites raising the issue of the direct environmental impact of these bulbs. Some of them contain a high level of mercury which causes concern when it comes to the disposal of the bulbs. To ensure that you're buying a truly environmentally friendly product have a look for the European Eco-label.