8mm, but unusually deep socket. Difficult to access. No: 1 almost impossible on my 2006 TDCI 1.6 Focus. Ford workshop did job for me but charged £400 labour.
Odd thing is that I knew the glo-plugs 2&3 were faulty - checked their wires from the fuse box - open circuit shown. But the warning light was not on.
Only when I had an intermittent fault with the gas-pedal sensor did the light come on and the motor went into limp. A local garage turned off the gas-pedal fault & confirmed the glo-plug fault (but could not turn it off). Then the motor ran OK.
Sounds like the management system computer program has got it's knickers in a twist.