Alice75 The 'Which' survey is very narrowly and strictly defined as We track the prices of up to 200 branded and own-label products every day over the course of a month to work out the average cost per item. The contents vary from month to month and might include anything from Andrex toilet paper and Lurpak butter to own-label bread and Heinz baked beans.
To be comparable a survey has to standard from survey to survey and year to year. Personal decisions of what supermarket to use will be governed by other factors, but distance from a shop, for example, is personal to every household. My convenient store may be very inconvenient for you and household sizes vary.
But as a broad brush national survey of prices for comparable products the 'Which survey is as good as any, and armed with that you can then incorporate it with all your personal factors to make the best decision for you.
I, for example, do all my shopping in Waitrose. It has in the past been considered expensive, but local Waitrose is a reasonable size, not too large, and is in the town centre, so any extra price I pay I consider worthwhile for being able to shop quickly and also be able to access all the other town centre facilities from shops, Bulding Society, hair dresser and even the local museum, while I am parked for free in the Waitrose car park.
But those other factors are personal to me and, even my next door neighbour may shop elsewhere for equally valid and different reasons.