Thanks to David (Braeside) who pointed me at it, I ended up buying a Minolta 24-105 zoom secondhand for £149. I didn't have a huge amount of hope for it, but it was from a reputable dealer, and it's very small and I couldn't possibly lose out if I sold it on ebay.
It arrived this morning, and apart from smelling a bit odd, it looks almost new. the zoom is slightly rough, and the manual focus as well.
It should be said at this point that I'm not a fan of cheap zooms . . I keep buying them because I want an occasional small / light solution, and I keep being disappointed.
Anyway, I took it out with the dog for half an hour this evening.
At 24mm the corners are soft - even stopped down . . . but only the very cornery corner, and there's some vignetting at longer focal lengths (again, even if stopped down).
But, as you can see, the bokeh isn't bad, it's not too prone to flare, CA is not too much of an issue, apart from the edges it's as sharp as a pin - even wide open. Added to which, it doesn't frighten the children if you point it at 'em from 10 ft (unlike the 135 f1.8).
It's not much bigger than the 50 f1.4, and it weighs very little, and it's a great focal length range.
So, Thanks to David for the heads up, and if you're looking for something light and cheap and cheerful to stick on your A900 when you're taking your dog for a walk and which won't break the bank or the back . . . this might just be it!
Worth noting that David Kilpatrick, on the Dyxum website Minolta 24-105
Reckons that it's unusable on the A900 . . .