very unlikely to be the glow plugs to be honest. very common problem on the early focus 1.8 tdci and can be caused by a number of things including wrong battery, cam possition sensor and even starter.

we changed prety uch everything on my mum's bar the starter and none made any difference including the glow plugs. was gonna change the starter too but my bro wrote the car off before i had chance.

it seams to me these are extremely sensitive to low voltage/low availible current to power the electronics if the starter decides to draw a fraction too much which is going to happen in winter when its coldest.

this fault also never throws up any fault codes from what i've heard.

good luck trying to fix it, most tend to live with it now i think as its just a load of hassle and expense to fix. still not as expensive as injector and pump problems though.