Yep Richard has good advice there. Plastic fittings and rubber hoses aren't the best of friends and usually the plastic part has moulding flash marks so a warm tighten will often cure the problem. The T piece just outside the header tank is a suspect part, they can jus totally fail and good practice is to replace it with an alloy option, easily obtained from ebay or car builder. If these initial efforts don't help then do a pressure test via the header tank cap, if all ok then suspect the header cap and replace, its a Ford item although the Iveco version is superior, same size fitment.
