Here's another vote for the Monarchs, I use the filberts mostly. They are a synthetic version of mongoose brushes, mongoose hair being used for exactly the properties you describe Alex, more springy than a sable but softer than a hog. Apparently mongoose is an endangered species now so W&N came up with a synthetic equivalent.
I did track down some real mongoose hair brushes at Cornelissen's -
http://www.cornelissen.co.uk - and, although I like them too, they're generally shorter than the monarchs. I feel a bit guilty about the mongooses (mongeese?) too.
Otherwise, I've found Pro Arte to be not worth bothering with, likewise Cirrus sables. Cornelissen do their own sables and hogs along with Roberson brushes of both types, all of which I've found to be very good, they keep their shape and don't loose hairs.
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