I think you should have one somewhere with an unvented hot water cylinder. They are usually a white tank fitted near to the hot water cylinder but I can't see one in your pictures. It may be built into the hot water tank or the boiler. In any case, you really shouldn't touch it as it must be done by someone qualified. Very dangerous.
The expansion is built into the top of the cylinder and you are allowed to replenish this yourself if you wish, its a very simple procedure and the instructions are on the label on the cylinder, to the right hand side of the tundish (the bit where the leak is) no tools required and very easy to do
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