It's up to you really. You don't have a strong argument, but with good adjudicator, you might win on the basis of the only clock visible on approach to the sign is the one in your car, which, whilst reasonably accurate is clearly not as accurate as the one used by the council, but the council have not placed such an accurate clock that they claim to use at the point of entry. There was a successful case some years ago where the adjudicator said there had to be some application of common sense, in view of the fact the motorist only has his own clock to rely on which may show a different time. Having said that, your are not a hugely strong ground with a four minute variance. Careful writing of the appeal emphasising the council demand absolute obedience to the times, yet do not place a clock to esure this. Let's face it, if they can afford the cameras they can afford a couple of clocks ?
Of course it's your money, so if you paid, the regulars on here, like me, will understand.