from looking at the pictures it would be very hard to see the corrosion, obviously the heat exchanger would cover the majority (if not all) of the corrosion points, they only way u would really know something was up would be if water was sitting in the bottom of combustion case when removed, or if combustion readings were high (only if hole in case). Pottertons are renowed for leaking internal and from drain cocks within the boiler, unfortunately you can't get the replacement casing as potterton no longer produce them, so it's new boiler!
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