Left in the cold over E.ON central heating insurance

My heating insurance policy was with E.ON, but new company HES says the boiler fault is not covered
Since 2006, I have been paying what I thought was E.ON for a policy to cover central heating problems. Last week my boiler broke down, at which point I discovered that the insurance contract, which costs £13.50 a month, had been taken over by a firm called Home Emergency Services (HES). HES responded promptly, but the engineer who arrived informed me that the problem wasn’t covered under t