exchangers and weather

While running the engine up after replacing the gearbox, the engineer commented that he thought my heat exchanger had pretty much failed....His reason.... "a small amount of water coming out of the overflow pipe on the fresh water circuit when the engine is run".... he seemed to think that sea water was getting in there....Now... working on the theory that he hasn't taste tested the water to see if it really is sea water (I hope not, as the fresh water system had antifreeze in it!), and given that I also topped the fresh water up to the top just before he started work and hadn't run the engine since, it got me to thinking..Here's my logic...... the only way that water could come out of the overflow is if it is being displaced by something..... and that could either be a] excessive pressure in the fresh water system, b] excess water from the calorifier system, or c] sea water leaking in....For a] the only 2 real causes of this would be either a blocked/restricted exchange core, or a blown head gasket passing compression pressures into the fresh water circuit... in which case, the levels in the header tank would drop as water was displaced from the fresh water system out of the overflow... (or obviously, overheating, which it isn't....)for b] checked by looking to see if coolant is entering the hot water system... but plausible...for c] I can't see this happening.... the fresh water circuit is under considerably higher pressure than the sea water circuit, and so surely displacement would be in the opposite direction?... maybe the process would reverse when the engine was shut down, but that wouldn't fit with the symptoms of an overflow when the engine was running surely?So..... my conclusion.....I think that when I topped up the header tank just before he worked on the engine, I put the levels above its natural level, and it is just overflowing a small amount of excess water, which will stop quickly (I added about 1/4 litre of water).... it is possible that the calorifier/hot water circuit is leaking into the system, but less likely than the prior reason... on top of this, the overflow wasn't leaking at all previously so it would be quite a coincidence that it failed at the same time as my gearbox....Anyway....if sea water is entering the exchanger, while it'll need fixing pronto, it won't stop us motoring for a few hours, so the collection trip is on.... or at least I hope it is!... the weather looks shocking...... forecast to be very windy (25kts average wind speed) this evening, and through tomorrow (Saturday) and then moderating Sunday morning....We can't get the boat out of the tidally constrained marina until either 0200 tomorrow or after that not until mid afternoon Sat.... not wanting to get up in the middle of the night we'll go for the afternoon option.... i'm hoping that the wind will have started to moderate by then, so that we can set off.... the sea state probably won't be very pleasant, but I guess we'll cope.... we can always wimp out and head into Ramsgate Saturday evening, and then continue the journey on Sunday if neccessary....