Combi boiler advice

I've got a Puma 100 boiler system - guess it's about 15 years old. Since I've lived in this house (10 years) it has always lost pressure (0.5 bar over maybe 3 months). A quick top up and no problem. However, recently it is losing pressure much quicker. Seems like about 0.5 bar over 2 or 3 days. I've checked the pressure release pipe from the boiler and nothing seems to be coming out of that. At this rate it can;t be a leak for the upstairs part of the system as I think it would be been visable dripping through the ceiling by now(!). I did open the boiler a few weeks back to clean the burner out and noticed green corrosion on the heat exchanger indicating a leak at some point. Wasn't wet but I guess it could maybe only leak when hot and the water is being evaporated off instantly hence the lack of dripping from the boiler. The other alternative is that it is a leak from pipers under the ground floor somewhere.
I'm hoping it's the heat exchanger theory - does this sound feasable? I have today injectwed some fernox F4 leak stopper so will see if it makes a difference.
As the boiler is now the age it is I'm going to have to replace soon. Can anyone recommend a condensing combi in the 100 - 120 btu range which has a good record, gives a decent hot water flow, good ecconomy etc. There are so many around I'm not sure how to start narrowing it down.
Finally, the current boiler has a 15mm gas feed. I understand that many of the new boilers of this size require a 22mm feed. Is this correct. Obviously, if I need to run a 22mm feed I will, but it will be a pain due to location so if it is not necessary all the better.
Finally, Finally(!) - ignoring the possible additional gas feed run what would the usual going rate be for a fitter to swap a boiler. It's located in a garage so all pipes etc are accessible and getting a neat finiash isn't essential like it would be in a kitchen etc. What can I expect for someone to do a straight boiler swap for a new model?
Thanks in anticipation...!
Cheers
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Further to my message below have now discovered the problem I think.... I went to check the pressure in the expansion vessel (CH) and water came out. Guess it needs a new one....?!

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