The Mayor of London is promoting a subsidised DIY loft insulation kit. You get everything you need for an average sized house (40 square metres) for ?49 (after ?50 cash back). For more information click here. The offer is open to anybody in Greater London and there is no expiry date listed. If you are over 70 or on eligible benefits you could get loft insulation for nothing. See here.
We recently had an energy audit and by far the biggest savings for the lowest costs were from boosting loft insulation.