Flue doesn't look sealed internally. If it were me I would be lifting it up out the back and replacing like for like (rgi). Your boiler stores the preheat water in a section of the expansion vessel so you can't cap it off and replace it but if it's not leaking you can leave in situ and fit another.
+1
The pressure in expansion units should be checked yearly when the boiler is serviced, my guess is it hasn't been serviced and the expansion unit hasn't been checked
i wish i could recharge it and safe money, there is water coming out the valve, also air got into my radiator after pumping air into the EV, i thought after that happening it must be faulty.
I went to screw fix and bought the supplies today, ill try and tackle the job later today. The fill loop is connected to the return, my plan is remove the check valve, fit an equal tee between the check valve and the pipe on the return pipe, then connect this to the EV which i intend to fit horizontally on the bottom of the cupboard. I hope this sounds fine.