Could very possibly be the boiler shell gone at a weld or bit of corrosion. Usually at one of the back corners of base of boiler.
Firstly have any auto vents, or pressure relief valve, etc, checked for leaks, or rain ingress from vertical flues.
It might be just bad luck, but I would check your system isn't corroding the boiler for some reason - no pumping over or absorbing air, if open system and that inhibitor was always in system.
Is it an HE (high efficiency condensing type), or standard efficiency? Also is it a basic heat only boiler, or system boiler (with pump, exp vessel) or the most complex - a combi boiler?
Why I ask that is the condensing boilers have more possible places for water to appear, including just the plastic condense trap and pipe, plus sometimes a faulty door seal can allow condense water leaks.
I assume you have a non condensing standard efficiency boiler. Unless you can have the boiler removed and blue casing taken of and shell welded, you will have to replace with a new condensing boiler with new flue installed to regs.
Personally I prefer Grant Vortex.