LONDON (Reuters) - British retailer Halfords (>> Halfords Group plc), whose boss Matt Davies will leave to take charge of Tesco's (>> Tesco PLC) UK business in June, posted a rise in Christmas sales thanks to strong demand for car parts, batteries and children's bicycles.

Retail sales at stores open for more than a year rose 6.8 percent in the 15 weeks to Jan. 9, the bikes-to-car-parts retailer said on Wednesday, ahead of an average forecast of 5.9 percent growth according to a Reuters poll of analysts.

Halfords said its full-year retail gross margin was likely to be in the better half of previously issued guidance, which anticipated a decline of 25 to 75 basis points, but that higher operating costs kept its group profit expectations unchanged.

The company is on average expected to post a full-year pre-tax profit of 81 million pounds according to Reuters data, up 11 percent on 2013/14. Shares edged 0.8 percent lower to 436.5p by 0930 GMT.

Car maintenance sales jumped 11 percent in the Christmas period as Halfords' fitting service for bulbs, batteries and blades reached record levels, completing 100,000 jobs in a week for the first time.

In cycling, where Halfords is expanding its accessories range and premium end offering to better compete in a 1 billion pound market, sales of children's bikes rose almost 14 percent.

Sales at the firm's smaller car repair unit, Autocentres, grew 5.9 percent in the period, its fiscal third quarter, ahead of expectations for 4.8 percent growth.

Chief Executive Davies has been credited with reviving the company's fortunes since joining in October 2012, boosting sales by improving stores and customer service. Shares tumbled on Jan. 8 when he announced his departure.

"I am gutted to be leaving Halfords," Davies told reporters on Wednesday. "It was only the scale of the Tesco opportunity and the opportunity to work with (Tesco CEO) Dave Lewis and the team that really pulled me away."

He will join Tesco on June 1. Halfords has not yet named his replacement.

(Editing by Kate Holton and Sam Wilkin)

By Neil Maidment

Stocks treated in this article : Tesco PLC, Halfords Group plc