moantreal Thanks for the tip on the hoses, I'll take a look at them while I'm in there.
I believe the source of the leak is a cracked thermostat - Maybe the bolt expanded as it rusted, cracking the housing and creating a slow leak.
As an aside, the job isn't too difficult. It just takes a lot more time in the winter...(Garage is insulated but not heated, so it was only 3C in there... had to take a number of breaks since I kept getting my hands wet/cold with coolant)