I've been a fool. Taking the exhaust duct out of the machine altogether, the air started blowing warm within 5 minutes. I was confused at first cos there was no obvious blockage in ducts and I could feel a strong draft through it. Eventually checked the external grille for the duct and discovered a dense mat of lint that was sodden from rain and dryer moisture. Zanussi is drying fine now.
Hope posting this helps someone else as dumb as me.
Jim McMillan
November 2008