Thomas Cook has agreed to the sale and leaseback of 17 of its aircraft. The deal will raise as much as £183 million as the struggling UK-based tour operator battles to improve its balance sheet. Thomas Cook chief executive Sam Weihagen said the deal, which was signed on May 5th, demonstrated the progress the company was making in strengthen its financial position.   “Today’s announcement demonstrates the progress which we continue to make to strengthen the group’s financial position, with the aircraft disposals providing substantial additional liquidity,” he said. The tour operator also releases a trading update earlier, revealing a fall in sales.   Summer UK mainstream bookings were down nine per cent Thomas Cook said, ahead of capacity reductions of 13 per cent. The group has 19 per cent less left to sell compared to prior year. “As expected, the first half seasonal losses have widened however, summer bookings have improved in recent weeks,” added Weihagen. The average selling price for a holiday was stable, up four per cent, and bookings for independent and specialist businesses are up 11 per cent.