Polti Vaporetto 2400 Leaking?

The above appliance, when heating up, starts to leak from the bottom - when cold water is put in initially this does not happen. I have been advised that this would be a washer ring on the water tank, when heated up expansion of the ring leads to the leak. My nearest Polti dealers are 140 miles away and collection will cost £34. If this problem is a leaking washer, the spare part would cost £2. So I would like to know how to go about fitting the washer myself. Any advice please?

Shropshire lass
April 2010