Yes. Shut off the supply of mains water to the house, and wait for however long you estimate it will take for the water level to rise in the F&E cistern. While waiting, monitor the level in the cold storage cistern and ll see if it drops by a propertional (i.e. smaller) amount. If it does then you'll know that water is leaving that part of the system and passing into the heating circuit via a hole or split in the cylinder coil.
No, but nor is it unheard of. Some of them fail after only one year. Some of them never fail.
You can't deduce anything from what's usual and what isn't. You can often save diagnostic time and effort by playing a hunch, based on probability of the cause, but you won't get it right every single time, and you certainly won't get it right by not testing for the simple and commonplace things, like the cylinder failing.
