Babcock International has been told to come up with a firm offer for VT Group by 5pm on 12 April or walk away.The company, which runs submarine bases for the Royal Navy at Faslane and Plymouth, has already increased the value of its proposal to 680-715p.Its first approach on 15 February valued VT, a provider of training to the Royal Navy, at 634p a share.Both approaches have been rejected as VT believes a combination of the two businesses would be "strategically unsound".It has reportedly offered a bumper payout to its shareholders worth £230m in an attempt to fight off Babcock.