We know they owed at least £150k to this individual from the case that led to their house being repossessed.
We know that there was a mortgage that would have had first call on any funds raised by the sale.
Given the sale price, it is unlikely that it did any more than cover the mortgage, and it may not even have done that.
Simply adding the standard interest rate to judgement debts for 12 years tells us that a £150k debt in 2013 would now be north of £375k. GIven that the Observer tells us that the debt was more than £150k by the time it went to court, and court fees, legal fees and the costs involved in repossessing and selling the house will have been added to that, it is almost certainly north of £400k by now.
On the question of legal action, it is too late for this individual to use the courts to recover the debt. The 6-year limitation period has long since expired. The debt still exists, but he can't use the courts to enforce it.