FAO John

Please Help

A new jet was put into the boiler on Friday and it was working fine until monday when it tripped. I then syphoned oil from the bottom of thetank to see if there was water in it. There wasn't. Since then it has started to trip more and more often. now it only runs for 15-30 mins and then trips. The boiler guy said its definitly getting water mixed with it. There is no water at the bottom of the tank. what do I do next? The tank is plastic...

cheers