I think there's a discontinuity around 25GW of gas generating. If we go over that, things get very expensive. The only way that happens is very low wind during the high demand period which is under the hours of darkness, and nuclear is diminished / there's nothing left in the pumped hydro stations. All of which we had yesterday.
Yesterday I think the only wind farm getting decent wind was Moray East in Scotland, and to add to the problems we even had to curtail the power from it (pay for it to reduce power). That was probably because the power couldn't be routed to where it was needed geographically. We need for transmission lines, which means pylons.
Edit: Had dogger bank wind farm been up and running (currently being commissioned - delayed from last year) this probably wouldn't have happened. We'd have had a couple of extra GW which would have been very useful.