Sainsbury's used to sell their Sainsbury's Fat Free Natural Yogurt 500g (white and blue pot) for about £1. Alongside that they sold their own Mary Ann's brand cheapo product for about 40p, which was presumably lesser quality.
Now, their own brand pot (identical packaging) is down to 40p (standard price, not offer) and the Mary Ann's has disappeared. I'm guessing they're now selling the Mary Ann's quality stuff under their own brand. It's impossible to know that for sure unless you're a Sainsbury's insider.
But, here's the point: A researcher recording prices for the ONS (or anyone else) is going to see that as a 60% reduction in price. So it shows up as food price deflation in the stats, whereas I suspect it's nothing of the kind.
IABU to think food price inflation is being understated because of stuff like this. We're all seeing our own shopping go up much faster than the headline rate.
Regardless, I think Sainsbury's has lost its way a bit. If the best they can do is advertise ALDI on every shelf then they're obviously out of ideas.