If you must sell up, check the loans on offer. Most lenders offer 105 to 110 per cent of the value of a new property. Abbey National allows up to 125 per cent to existing customers, Bank of Scotland up to 120 per cent loans to new customers. HMC caters for new customers bringing negative equity with them.
If necessary, consider letting out the current property, and renting a new home for a while. You should use available capital to reduce your loan.