You could be liable for half the tax in the sense that his tax liability goes into the financial pot and so decreases the amount of his assets. The courts are also part of the government and take a dim view of people not paying tax so you should argue that although the tax (which would have been payed anyway had your stbx been diligent) is a relevant liability, any interest, surcharge, fines and penalties should be borne by him alone.
Lync. Tax is a debt and it depends when the was acrued. Anything debt after seperation is the persons who run it up resposibility. In other words if you seperate on a momday and on tuesday go and buy 10 new outfits for £1000 each its your debt to service. Its not joint. But debt goes into the pot like Vail said. It lessons his disposable income as he must service this debt. The very good news for you looking forward is that the tax man wont chase you for it. That is good news.
If he had the money and spent it thats asset sheding and looks bad for him. But at the end of the day it all comes down to the judge. Its like fliping a coin or like crap shooting. Very variable. C