The board of Britain’s largest private hospital operator has backed a buyout proposal worth £1bn from its second-biggest shareholder, a hedge fund manager known as “the Rottweiler”, sending its shares soaring by nearly 50%. Spire Healthcare, which owns the Claremont hospital in Sheffield and St Anthony’s hospital in south London, said it had received a non-binding proposal worth 250p a share from funds advised by the activist investor Toscafund Asset Management. Toscafund was founded in 2000 by Martin Hughes, a longtime City figure who has been at forefront of many takeover situations, earning him the nickname “the Rottweiler” for his aggressive approach. Spire said: “The possible cash offer is at a value that the board would be minded to recommend unanimously to Spire Healthcare shareholders” if a firm offer was tabled. Its share price – which had hit a five-year low at 142p in March – jumped by 47p to 221p on Thursday, giving the company a market capitalisation of £892m.…