Both you and your ex (or either of you) can be chased for the full amount of the shortfall (i.e. the amount by which the outstanding mortgage & other costs exceeded the sale price of the repossessed house).
Because it is a mortgage debt the statute of limitations is 12 years, so you are not there yet. However, there is a voluntary code operated by members of the Council of Mortgage Lenders (CML) that applies a 6 year limit in most cases. You should be able to find details on the CML site to see whether you come within the conditions. This may mean (if your lender is a CML member) that you are in the clear.
But if the lender obtained a money judgement (CCJ) from the Court when they repossessed, the 12 year limit does not apply - there is no limit in this case. The voluntary 6 year limit may still apply, but I am not certain about it.