Why don't you try asking the staff at your local store when they do their big reductions? I have always found that the staff at mine are more than happy to tell me 
FWIW, when I was at my Tesco yesterday at around 3.30pm, most things were down to around 50% off, except some bakery goods (2 Eccles cakes for 30p, anyone - yummy!) & some fruit that they clearly had a glut of (at least 10 boxes of nectarines, so a punnet was selling for 20p) & I couldn't see any meaty bargains. I have noticed that they tend to reduce earlier on weekday evenings, at around 7pm instead of 8pm, so I may pop down there tonight with the DSes before their bedtime!
Also, my local store staff have told me in the past that reductions will be greater/more plentiful when the weather is bad so nobody is likely to venture out to the shops, or there is a big sporting event on which everybody will be watching (eg World Cup football), so again not many people will be shopping.
If you have a Sainsbury's, we went to our last Sunday at around 3.30pm & got all sorts of bargains - large pieces of Brie for 25p, organic beef for 99p, sandwiches for the next day's trip to the Olympics for 20p & lots more besides!
HTH