Don't know Giles Coren, but just off the top of my head, it seems to me that multiple visits - leaving fairness out of it altogether - allow one to assess/compare for one's readers' benefit:
* a greater percentage of the menu
* a wider range of specials (or just whether there is a wide range)
* how they handle slow times vs. rush times
* lunch vs. dinner
* groups vs. solo diners
* walk-ins vs. reservations
* consistency of the same dishes at different times
* consistency of staff training/performance
And perhaps any number of other things worth knowing.
But then, I'm:
* American
* thick and getting thicker all the time
* a frequent writer of drivel
Just noticed that in the OP his name is linked. So, intrigued, I clicked.
My gosh what a lot of 'tude one has to wade through just to learn something (or not) about a restaurant. Jaw-clenching, headache inducing toxic levels of snarkier-than-thou-ishness which might be tolerable if it seemed natural and effortless, but, alas, seems all too drearily straining for attention and angry letters from readers to validate his column space. So glad he's over there. Give me Antonius, Hammond, Sula, my pal Will, Wiv. et al any (and every) day.
"Strange how potent cheap music is."