Interesting topic... I too have a similar problem.
I had a coolant leak, was pouring tons of water. I replaced the HBV with one off ebay - cost me £13.99

but still have a much slower leak. After a 20 mile trip if I immediately pop the bonnet I see a bunch of steam floating around behind the plenum.
There is a slight chance that I punctured the hose on the offside of the HBV during fiting (I'm a monkey when it comes to spanners and engines and stuff like that lol) but I still have not had a chance to investigate further.
I am now concerned about this 'transfer pipe' as a possible cause of the issue as my car is occasionally starting up lumpy every now and then (sorts it self out after 10-20 seconds) and makes me wonder if my dispack is getting soggy.
The guide for the HBV replacement was really useful, is there a guide for the transfer pipe? or any of the other common causes of coolant leak?
Thanks,
Ben