Right, I've just gone and tried:

20x20mm square of 0.8mm copper, borax, hard solder chip (full thickness, not rolled down at all) Nimrod T75 butane torch - slow, but got there eventually. Solder melted & flowed, although not all that far - I suspect the flux was probably used up by that time.
40x40mm square of 0.8mm copper, borax, hard solder chip, Nimrod T75 + another butane torch, copper boxed in with ceramic block to retain heat - gave up after 5 mins. Flux was black, but there was no way the piece was ever going to hit solder flow temperature.

Refluxed, hit with the Smith Little Torch (#6 tip) - 5-10s for the solder to flow.

Butane torches are great little things for small jobs, but they don't really have the BTUs to get reliable joints in larger pieces. If you don't want to jump up to something like a Sievert, then one of these would probably do well - http://www.diy.com/hand-tools/tp2062.../196618_BQ.prd Usually I'd suggest some of the Bernzomatic range, but B&Q don't seem to do them any more.