* Relaunch is biggest investment in "finest" for 15 years
* Investing "tens of millions of pounds" in relaunch
* Forecasts "double digit" percentage rise in "finest" sales
LONDON, Oct 7 (Reuters) - Tesco, Britain's biggest retailer, has relaunched its "finest" premium food range, aiming to tap the industry-beating sales growth enjoyed by upmarket rival Waitrose and reverse years of market share losses.
The world's third-biggest retailer behind France's Carrefour and U.S. number one Wal-Mart is 1-1/2 years into a 1-billion pound ($1.6 billion) turnaround plan in its main British market, where it has been losing ground after focusing investment on a rapid expansion overseas.
The company said on Monday it had spent "tens of millions of pounds" on revamping its "finest" range and expected a "double digit" percentage rise in sales from products including a trifle made with Amontillado sherry syrup, Aberdeen Angus beef steaks and Anglesey pure sea salt.
While many Britons have been cutting back on grocery spending in a struggling economy, fuelling a surge in sales at discounters Aldi and Lidl, many have also been treating themselves at John Lewis' Waitrose chain.
The latest industry data from Kantar Worldpanel show Waitrose growing sales at 9.7 percent year-on-year in the 12 weeks to Sept. 15, compared with just 1.9 percent at Tesco.
"This isn't about taking on Waitrose," Tesco's UK managing director Chris Bush told reporters at the relaunch of the 1,500 product "finest" range, currently worth 1.4 billion pounds in annual sales.
"This is us continuing on our journey on improving our food offer that started last year with the relaunch of (value range) Everyday Value, the continual improvement of (the mid-tier) Tesco brand and now the relaunch of finest," he said.
The relaunch is being supported by sponsorship of Downton Abbey, currently Britain's most popular television drama series, and by an advertising campaign that starts later this month.
Despite heavy investment in new products, staff and revamped stores, Tesco last week posted flat underlying quarterly sales in Britain, while plunging profits in mainland Europe added to pressure on its struggling overseas business.
Fifteen years after it began as a selection of ready-made meals, the "finest" relaunch includes 400 new products, with three quarters of the range either new or improved and 200 products removed.
Shares in Tesco were up 0.6 pence at 362 pence at 1500 GMT, valuing the business at 29.2 billion pounds.