That's the one @DavidH1983, that will at least identify where the issue is which is what you are trying to achieve? Yes it would be a good idea to top up the inhibitor. Isolate 1 rad, drain it and fill it with 1L of good inhibitor, cant go wrong.
I would be very surprised if the expansion vessel/prv was the issue here on a newish system, unless it was an out and out failure, if the pressure doesn't increase to 3+bar when the heating is on, then that's not the issue.
Blue roll - best thing to use to check for leaks as it changes to dark blue with any water on it.
Great shout on the blue roll. I have some here at work I can pinch.
I would be surprised if it was the boiler too but these things can happen I guess. I have yet to sit by the boiler and turn the heating on to test this but will do this at the weekend. Am I right in thinking I should see roughly a 10% increase in pressure?