I think I have found mine as well....
Having finally got access to all the ground floor pipework (lifting floors etc), I found - erm nothing!
So today while the heating was on setback I decided to make some checks on the boiler itself. Lifted its pressure to about 1.5 bar, and then turned its power off. I thought that I would also check the condensate drain. Interestingly - normally its spotlessly clean and full of clear water. This time it looked decidedly orange and slightly scummy. So I emptied and cleaned it and put it back, then left it alone for a couple of hours. Had a look again and its now 3/4 full of water. Emptied again, left another hour or so, and verified that the system pressure had fallen a little. (I replaced a rad vent plug with a larger pressure gauge to be able to more clearly see the difference without the aid of the boilers electronic gauge).
So the good news is at least the plumbing is ok, and I have not got to fix something in some awkward difficult to get at place. The less good news is it looks like the primary HEX mush have a small internal leak.
Quite how/why that happened on a 7 year old boiler on a well cleaned system that was always treated with inhibitor I am not sure. I suppose until I look in the combustion chamber its hard to say what is going on.
Replacement HEX seem to range from £120 for a reconditioned one, and new ones ranging from £220 - £450 depending on where you get it.
Now I know its not leaking water into the house I may just slap in some more inhibitor and leave it until the warmer months!