By Russell HottenBusiness reporter, BBC News

Porsche is warning UK customers they might have to pay 10% extra for cars delivered after Britain leaves the EU.
The German firm wants buyers to sign a clause agreeing to a potential tariff, a move Porsche said is “precautionary”.
Porsche’s owner Volkswagen declined to discuss if some of its other brands, including Audi, Lamborghini, Skoda, Bugatti, Seat, and Ducati might follow.
A 10% surcharge would see the cost of an entry-level Porsche 911 rising from £93,110 to £102,421.
The company’s Macan sports utility and Boxster models start at about £46,000.
Stuttgart-based Porsche said in an emailed statement to the BBC: “As one potential outcome of the Brexit negotiations, there is a possibility that a duty of up to 10% may be applied to cars imported into the UK by us after March 29.