UK grocer Waitrose said today that its Pick Your Own Offers (PYOO) loyalty initiative is helping boost sales.

In its weekly trading statement the supermarket revealed that it has now signed up 850,000 of its myWaitrose members to the scheme – a programme that allows people to select ten of their favourite products which they will then receive a 20% discount on every time they purchase those items.

Customer loyalty patterns in grocery retail have shifted in recent years, with new industry players joining the market and undercutting and price-matching becoming the norm.

Recent research from industry charity IGD showed that only 10% of shoppers buy all their groceries from the same retailer and 22% are highly promiscuous, buying no more than half their groceries from any one place.

Speaking at IGD's annual convention last week, the organisation's CEO Joanne Denney-Finch said: "There's no single solution.

"A quarter of shoppers told us nothing would induce them to buy more from their number one store but it will start to change as a natural fault-line opens up in grocery shopping, between the regular staples picked up by reflex and those products selected each time."

Morrisons chairman Andy Higginson, who also spoke at the event, said that grocers no longer secure shoppers for the long-term, adding that they need to "earn their loyalty every week".

Nevertheless for Waitrose, PYOO and a half-price promotion were said to be driving forces in the company's 2.7% year-on-year increase in sales for the week to 10 October 2015, as the grocer generated sales of £123.87 million over the seven-day period.

Other factors played their part, too.

David Jones, supply chain director at Waitrose, commented: "The unseasonably warm weather for October was influential, too, with ice cream sales up nearly 5% and strawberries growing by an incredible 45%.

"The excitement of the Rugby World Cup is captivating sports fans and influencing sales. As supporters geared up to either celebrate or commiserate, Australian and South African wine sales increased by 38% and 19% respectively. World beer sales were up 25% and canned ales and stouts were up 18% compared to this time last year."