One way to get them out may be to heat up the flue to the stage where they leave because it's just too hot to stay there. A safe (but inconvenient) way to do this is to get an electric blanket, set it at the hottest possible setting, wrap it around the flue without blocking their exit route, plug in the lead and turn it on.

If their entrance is not near the top of the flue you may have to poke a temporary block (tin lid nailed to the end of a curtain rod?) down to just above their exit route, and then just heat up that section below them.

Don't do it in the rain, though. And a certain amount of noisy banging on the outside of the flue may help them decide early, once they're warm enough to get moving.

Cheers, Rowan