If the cylinder is leaking you should have signs of a water leak on ceilings floors etc.
It maybe worth checking yourself.
As for the cylinder, "servicing" I have heard this cop out before, mainly regarding Barratt's old five year warranty, often not worth the paper its presumably written on.
Quite how a cylinder, with no moving parts, can be serviced I don't know!
It can only be visually checked and the water topped up.
I would assume that adding corrosion inhibitor such as Fernox who be part of a boiler service.
So a "service" of a water cylinder would not have prevented it leaking now.
All you can do is make this very public online naming the builder, warranty provider and cylinder Manufacturer.
Here is a great forum for
consumer advice.