Waitrose’s sales growth has been the envy of other food retailers for some time - but its profitability, this time last year, was rather less so.

Operating profits for the first half of 2011 had fallen by 14% to £110.2m after major investments in convenience stores, the online business and the Channel Islands. It was the only black spot in an otherwise exemplary set of results.

This time around, the half-year results show Waitrose has continued to perform well against its peers in terms of sales, up 2.2% on a like-for-like basis - and it’s back on track in terms of profits too.

While some of the remarkable 28.9% increase in operating profits reflects how heavily Waitrose invested last year, it’s not as if 2012 has seen spending slashed to zero.

The upmarket supermarket has invested heavily in price, most notably extending its price-matching with Tesco to all branded groceries. It’s also increased promotions, promising to run at least 1,000 a month. The results are clear to see in the Grocer Price Index, which this month showed that its prices were only 1% higher than a year ago.

Seen in that context, the profit growth published today is even more impressive. At 5.4%, profit margin is well ahead of the 4.8% industry average revealed by The Grocer’s Top 100 ranking of UK grocer retailers last week - a finding referenced directly today by MD Mark Price.

All in all, it’s no wonder Price sounded so happy with life when he spoke exclusively to us last week about his plans to double Waitrose in size. Trial and error, he said, had been key to recent expansion. But at the moment Price shows little sign of putting a foot wrong.