thanks everyone - helpful
Oggsfrog - if you decide to go for it I think all is not by any means lost
I don't think age would be a factor. It might have been in the past but the rules changed recently and I don't think 40 is a problem.
Also, where I am (North London and I guess this is common) they have a rule that there must be 2 years between a child in the family and the adopted child. And they don't seem wild about one adopting a child older than a child in the family so your situation would be ideal. And they're dead keen on people adopting sibs.
Spect you know all this so I'll shut up but if you decide anything, with you all the way whichever way.