If the boiler is dropping in pressure after being isolated from system, then this tells me the boiler is leaking. Leave it all to a heating engineer as carrying out work on these type of systems is dangerous and illegal if not gas safe registered.

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Is your pressure relief valve letting bye!
Possible cause of overfill?
Is the expansion vessel at fault?
Is there a problem with.heat exchanger?
The list goes on,?
You need someone who has the knowledge for fault finding,
Good engineers do not become good overnight!!
Good luck.

A boiler fitted on the 10/10/2019, would still be covered by the manufactures warranty any problems with the boiler will be repaired by them, providing the installation meets current standards and best practices.

If the boiler is isolated and it’s not the pressure relief valve letting by, it will be worth keeping the boiler isolate and turned off and disconnect the condense pipe and wait to see if water comes out of the trap. I.e leave a bowl under it, if water appears even tho the boiler has not been on it would suggest that there is a leak internally on the heat exchanger

You state there is water dripping out of the flue, is the boiler running at this point? If it is the flue may be incorrectIy lnstalled, however this will not cause a pressure drop. If the boiler is not running then the chamber is filling up the boiler is then faulty and you have a warranty issue. you have isolated the boiler and there is no visible signs of water around the boiler, the chances are there is a leak in the system, however recharge the boiler and fit a container over the 3 bar pressure relief copper outlet,making sure there is a suitable air gap between the pipe and container check in the morning to see if the container has any water residue in it ( allowing for rainfall)! If none present then there is a leak in the system and it will require professional pressure testing to find the leak. Good luck!