Mystery slow leak

I've been having an on going problem with my car for a year. I'ts now done 38,00 miles so i know they are prone to the HG going.
ok, there is a slow leak - nearly boils dry after about a month or so (although i dont let it get that low now i know about it!) there is no visible leak anywhere, taken it to the garage three times and on the third occasion they found a cracked radiator which we replaced. The leak is still there. Apparently the HG is fine, (had two separate opinions, one from a helpful rover enthusiast i bumped into last time it overheated) pressure test was fine, no fumes or anything coming form the exhaust, no oil in the water or water in oil, i check regularly. So no idea where this water is going, and neither do they. Also to add to it, today after i parked up and turned off my engine, the fan kicked in, and prior to turning off the engine the air con suddenly started pumping out cold air instead of hot - yet the engine temp showed as completely normal the whole time. Any chance I've got a dodgy thermostat as well, or could that even be the cause of the whole thing??
Hoping someone can help, I'm getting sick of having to check the coolant level every time before i set off anywhere and having to carry a spark tank of water in my car just incase!