“… [T]he purpose of section 28 of the [Building Societies Act 1962 ] was to put the borrower on notice that the basic security over his house was not thought by the building society to be adequate and that, in the event of default, he might become liable, not only to the society under his covenant for payment, but also to the rights, by way of subrogation, of an institutional guarantor who had been required to make a payment to the society in respect of a shortfall. … It is not, I think, helpful to attempt to categorise the arrangement as a ‘guarantee,’ an ‘indemnity’ or ‘insurance’. There are features which suggest each of those categories; but the arrangement does not fit readily into any one category rather than another. The security which the Mortgage Indemnity Guarantee is intended to provide is the covenant of the insurance company.”