A Dow Jones columnist has aired the views of the NBNK Investment camp on National Australia Bank's perceptions of the value of its Clydesdale Bank subsidiary in the UK.NAB is holding on to an "unrealistic" valuation of its business, Dow Jones reported, given the rise in impairments and the deficit in the pension scheme. The latter runs into hundreds of millions of pounds, the columnist's source told Dow Jones.NAB has paid in £530 million over recent years to help cover pension scheme losses, which is more than half of the additional capital contributed by NAB to its UK subsidiary over the period.NBNK has focused its present interest in buying UK banking assets on the Verde branches being sold by Lloyds Banking Group.Plenty of conjecture has emerged in the UK media over the last 10 months in relation to NAB's negotiations and options over Clydesdale Bank, including talk of "good bank, bad bank" solutions.There is some pressure on NAB's management to outline a realistic plan for its UK business by the time it reports its interim profit on May 10, a deadline promoted by NAB back in February.