Duffy scores the year's highest tally solely through digital sales.

After crushing Nickelback's aspirations last week, Duffy dashes further ahead at the top of the singles chart with Mercy increasing sales by 32.8% to 50,962. That is the highest number one sale thus far in 2008 - and all digital, apart from a minor leakage of 17 CD and seven-inch vinyl sales apiece. With the physical formats of Mercy officially out today (Monday), it is likely to increase its tally yet again next week.

Mercy was only the third single to debut at number one prior to physical release since rules changed to allow this to happen two years ago, after Gnarls Barkley's Crazy and the Rihanna/Jay-Z collaboration Umbrella. From the North Welsh town of Nefyn, Duffy is the first female solo artist from the principality to have a number one single since Bonnie Tyler's Total Eclipse Of The Heart was top in 1983, more than a year before Duffy was born.

Duffy's strength is tough on H Two O feat. Platnum, whose What's It Gonna Be elevates 7-2 on sales up 138.8% at 43,089, following its physical release; the highest sale for a number two single this year. It easily beats Nickelback's Rockstar, which slides 2-3 on sales of 27,965 - a 14.5% dip week-on-week. It is only the second time in 22 weeks that Rockstar has not increased its sales.

Meanwhile, the chart's fastest climber is Just by Mark ...