If I was still at my old work I could tell you specifically if it was a grid-level issue, but my money would be on something more local like a substation.
Grid brown-outs are quite major events, either caused by consumption in excess of system capacity (unlikely at the mo'), a generator failure or a big failure of distribution. If the system is pulling more than is available, the voltage can drop and the frequency becomes unstable (tolerance is +/-0.2 Hz on 50Hz from memory) which manifests itself as a 'brownout'.
I worked specifically on something called the 'balancing mechanism', which is the market that matches electricity demand with supply across the national grid in real-time as electrical energy cannot be effectively stored in large quantities, for the time-being at least.
Now if you want an explanation of the P142 electricity pricing mechanism........