ok.
First thing, remove NRV from expansion pipe. It will do nothing for you and will cause problems if it sticks. I.E it is dangerous and a big no no.
Things to remember are that 1 bar pressure = 10 metre head. How high is F/E tank from top of boiler. this will determine pressure on gauge. On a vented system, you should be getting about 1/2 bar reading on the gauge. Again this depends on position of tank in relation to position of gauge, Don't worry too much about pressure for the moment.
Do you think you have a leak?
If you seal the system, it will only leak faster due to higher pressure.
Try this first.
Tie up ballcock in F/E tank and leave it for a week, observe water level over this time. If it drops alot, then you have a leak.