isolating the leak where combi slowly loses pressure

[wayneyoung123]

My boiler is losing about 0.1bar of pressure a day..then i have to refill and reset the boiler to work.
Work already been done
Insurance company has replaced the PRV valve despite there never being any water coming out of the pipe. He respressurised something with a footpump. Then the company has refused to come back because the boiler is very bad condition, hasn't been serviced regularly and needs a powerflush! I have given up with them!(after 3 hours of pointless arguments)
I have just had a guy put in some sentinal selant into the system so hopefully that will help.
There is no dripping in the boiler cupboard itself.
How do i find the leak?
Can I isolate the boiler from the pipework by turning off the in/out flow at the boiler? If the pressure still dropped then i assume it would point a finger at the boiler losing pressure internally somewhere rather than in the pipework? Not sure how to find the relevant taps/valves to turn off this?
Can i turn off the radiators in the back half of the house...meaning that the boiler pressure gage can only be effected by a drop in pressure/leak in the rads in the front part of the house?. And then reverse this to see if it is the rads in the back half of the house.? How do i do this? Some of the rads have a dial which will turn off the water to that specific radiator but how do i stop the flow to subsequent rads. And how do I know what the layout it of the rads...ie. how they are linked together?
Boiler details
wocester 9.24 boiler - 17 years old
roughly 10 radiators/ground floor

 

[wnjswozo]

check all rad vavles,recheck boiler,the plumber who has put ls1 in,is on the right track,your boiler is coming to the end of its working life,start planning to replace,you,ll be lucky to trace such a small leak.my bet is expansion vessal,or some inaccessable compression fitting weeping,underfloor

 

[payney1974]

you can isolate the boiler and leave it overnight to rule the boiler out but at 17 years old and bad water quality the isolation valves will more than likely leak after and they are awkward to replace, i presume the engineer checked the auto air vent? to be honest though i agree with what wnjswozo said about replacing the system soon, as 17 years is a grand old age!!