Canny shoppers have been able to make big savings on multipacks of colas in supermarkets in the past year - a trend contributing to increased pressure on the impulse market.

This week, Coca-Cola Enterprises admitted sales of cans of soft drinks in general had gone a little flat in c-stores, prompting the launch of 45p 250ml cans of Coke to offer a lower price point than the standard 330ml.

The pressure c-stores are under is plain to see from the volatile cola price movements in the mults over the past year.

Although the average price of a single 330ml can of cola (brand and own-label) has increased marginally, from 56p to 57p, in the big five supermarkets over the past 12 months, the price of eight-packs has been as low as £1.99 - working out at just 25p per 330ml can [].

Prices have not just been low but also highly variable. The average price of a 330ml eight-pack (brand and own label) was £3.39 last July, dropped to a low of £2.62 in November, rose again but fell back down to £3.13 last week.

Changes in promotional activity have played a big role in shifting multipack prices. Coca-Cola, which reined in price promotional activity in 2012 as it focused on the Olympics, has contributed to this with a major push in 2013. Although the total number of promotions across the cola category is actually down slightly year-on-year, deals have become much more aggressive - the average depth of cut is now 27.9% compared with 21% a year ago.

Away from multipacks, however, and taking into account the entire cola market across all packaging formats, cola has actually become more expensive on a per-litre basis, with the average volume price rising by 2.4%, from £1.27 to £1.30, year-on-year. This increase has been driven by brands, as the average own-label volume price has remained static at 46p per litre.

In the case of single two-litre bottles, the typical unit price has risen from £1.24 to £1.30, and currently ranges from £1.98 for leading brands down to just 17p for 2-litre budget diet cola.

Prices have also increased in the case of the 250ml cans introduced by Britvic for the Pepsi brand two years ago. The average price of a 250ml Pepsi multipack - ranging in size from 12 to 24 cans - was £4.98 a year ago but is currently £5.43.

A Britvic spokesman said the company did not set retail prices to customers, but only recommended a retail price.