November's Grocer Price Index showed a small month-on-month fall in retail prices of 0.1%. But anyone looking to explain the fall would do better to look to Christmas than commodity prices.

Two categories drove the fall in prices: alcohol and snack food (biscuits, crisps and confectionery). It's no coincidence these categories are the main targets for Christmas promotional savings.

The level of promotional activities in these areas has been huge. Last week there were nearly 600 featured-space deals on beers, wine and spirits, making up 30% of all promotions. Deals on snack food followed close behind.

Away from the festive discounts, the pricing news is less merry. The average price of fruit and vegetables was up 5.9% in November, caused by several key lines switching from UK harvest to imports while the pound continued to plummet, driving up the price of imports.

The price of dry groceries - staple ambient lines - also increased in November, up 2.2% on October prices. Lagged price increases on several canned food lines were a big driver of this. There has been a 53% rise in the price of own-label baked beans, for instance. Suppliers blame increases in the input price of tomato paste, haricot beans and the material used to make the cans, the price of which has not mirrored drops in the headline commodity price of steel.

Soft drinks, the only category cheaper this year than last, also had a price rise in November, caused largely by a shift in promotional activity for Christmas.

Deli goods and health & beauty products also rose slightly in price, though bakery and dairy lines were slightly cheaper month-on-month. The overall effect led to a slight increase in annual food price inflation, up from 13.9% in October to 14.1% in November.

There was a dramatic shift in retailers' monthly inflation rates, as Asda and Morrisons' prices, which had fallen or remained flat in October, rose in November.

Asda's prices were up 0.7% in November, against a 1.5% fall the previous month, while Morrisons' prices remained stable through October but went up 0.7% in November.

Prices continued to fall at Tesco, albeit at a slower rate than in October, down 0.2% compared with 0.8% the previous month.

But Sainsbury's, which was the only retailer to have higher prices through October, with a 2% rise, dramatically reversed its trend by ramping up its promotional offer. Sainsbury's prices fell a hefty 1.9% in November, the biggest month-on-month drop of any of the big four retailers this year.

Asda had the lowest average item price of the retailers, with a typical £1.99 price tag, well ahead of its nearest rival. Tesco came in second with a £2.07 tag, followed very closely by Sainsbury's, a fraction of a penny behind but also on £2.07. Morrisons was the most expensive, with a typical £2.09 price tag.