It may have been wishful thinking in the past on the part of sell-side analysts and the media, but British bank Northern Rock has sometimes made the list of targets that National Australia Bank might aim to buy.Were NAB to still harbour any interest in the UK's fifth-largest mortgage lender, now may be the time to step forward.News of the acute liquidity position of Northern Rock at the end of last week sparked a run on the Newcastle-based lender and further hammered a weak share price.Northern Rock is primarily a mortgage lender that has sustained rapid growth over the last decade by relying on wholesale finance from banks and through securitisation of mortgages. With asset-backed markets more or less frozen and the cost of inter-bank funding escalating the bank last week negotiated an unlimited lending facility from the Bank of England.While the bank said it was yet to draw on the Bank of England loan and it, and the central bank, and Treasury ministers, and the banking and mortgage associations all insisted Northern Rock was solvent, plenty of customers didn't agree. Queues formed down the street at many branches, which had to extend trading hours. Media reports put aggregate withdrawals on Friday at in excess of 2 billion pounds.Northern Rock refused to meet some withdrawal requests (for online accounts, for example) and called police to evict determined customers in some cases according to British media reports.Northern Rock has liabilities of 110 billion pounds, net assets of 3.3 billion pounds and a much reduced market capitalisation of 1.85 billion pounds as of Friday.This is about four times the size of NAB's banking business in Britain and Europe, conducted through the Clydesdale and Yorkshire bank brands.Management of Northern Rock is already talking of changes to its business model and fostering talk of a break-up or a takeover bid.Speculation in British media identifies Lloyds TSB as the favourite to take over Northern Rock. Of the other clearing banks two (Barclays and RBS) are trying to buy ABN Amro and HSBC has suffered its own localised shocks from the US sub-prime rout through Household Finance. Various foreign banks and private equity buyers (with a track record of buying troubled Asian banks) also feature in the speculation.Of the other mortgage lenders in Britain that NAB is sometimes imagined to have on its wish list, Alliance & Leicester has a market cap of 3.8 billion pounds and Bradford & Bingley a market cap of 2.1 billion pounds.Local speculation, meanwhile, continues to tip NAB as a buyer of Rams Home Loans - though only of the $16 billion loan portfolio and the customer list and the not the whole business. This rumour might explain the 14 per cent rise in the Rams share price on Friday.