I've now had to admit defeat and give up on the cantilever brakes, much as I like the look of them and in keeping with the look of the bike, they certainly don't earn their keep in the stopping department!
All the bikes I've owned for at least the last 10-15 years, including two I still own, have hydraulic disc brakes and I've become used to not worrying whether I'll stop when required, that has not been the case with the cantilever brakes, I'm sure they're as good as they were designed to be but at 6' 5" and 250lbs, it isn't enough.
I've just ordered a set of V brakes (not as easy to find nowadays, unless you're happy with a Chinese V brake shaped thing) and the appropriate drop bar levers, that will be my job for this weekend...