Jane discovered that the £3,000 had turned to ash/ashes OR Jane discovered that the £3,000 had become ash/ashes. 'Ash' is one of those strange words which doesn't really have a distinction between singular and plural - in general 'ash' is used to mean the residue after something has been completely burnt, e.g. cigarettes, but 'ashes' is often used when referring to the remains of a cremation.