Ofgem’s Electricity Capacity Estimates was released recently. The data is from National Grid as well as Ofgem’s own research.
The assessment states that if there are harsh winters within the next two years, lights could dim for up to NINE hours. It also states:
[The margins of electricity usage] are still expected to drop to their lowest level in 2015-2016 resulting from closure of older power stations. After this, the margins are expected to improve as new power stations are introduced.