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If I'm self-catering on a job, I get by on water, sandwiches, miniature cans of fizzy drinks, and Haribos Tangfastics (other forms of confectionery are available, although I don't understand why). It basically becomes a day of sugar-boosts, which isn't something to do on a regular basis, but once in a while won't kill you and it helps me get the job done.
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Incidentally, I eat when the wedding party eats. I make a point of NOT taking photos when food's on its way into, or in, someone's mouth. It does not make for memorable photography as Ed Milliband will testify.
Recently, in advance of their wedding, a couple asked the same question and I gave the same answer. As they passed me on their entrance to the wedding breakfast, the groom slipped a bag of Tangfastics into my hand. It was nice of them to remember, and very welcome - but the icing on the cake (so to speak) came a little while later when the Event Manager told me there was a main course waiting for me in the bar. Cheers, Helen and Trevor.
As I say: not necessary, or expected. But always appreciated.