[kiloran]

Just want to share my experience here - and appreciate that something as big as an HG job is something you have to plan in and budget for, but mine had been leaking I think for the best part of the year before it finally failed catastrophically. I was doing the same as you are with regular top ups of mixed coolant and the failure was on the OSF corner of the block above the water pump. Because it had been left for so long it resulted in corrosion/pitting to the block face, which can end up meaning it turns in to a much bigger job.