At the risk of being shot down in flames (again), I offer the following, purely as an observation.
When we retired here to France, we had central heating installed by a local artisan. NONE of the joints were soldered – all were brazed, most being formed by swaging one pipe onto the adjoining one, then brazing with an oxy-acetylene kit. Never had a leak. I don't know what you are going to join – I assume it's the boiler, etc, nor do I know the thickness of the materials, but maybe you could consider this method. I stumbled upon a company called Harris, in the States, who specialise in solder and brazing materials. They do an interesting document on their website which details techniques and products, some of which are brass coloured. Their web address is:
http://www.harrisproductsgroup.com
There is another product, also brass coloured, called Filler Pro – Google it – again from the USA used in cycle frame construction. You'll need a mortgage to get some, judging by the prices on the web site, though!
(Climbs back into box to dodge the flak)
John