Looking on the Sage UK site I found the following Developer SDK.
Upshot is that you need to use .Net if you want to use the SDK.
Problem is that the SDK is only available under the Developer programme which starts at £1500: Here's the brochure.
However the developer programme does give you free copies of the Sage software for development purposes, so I can see the benefits if your business is Sage integration.
Another option is an addon for Sage which is sold by Sage for £299
http://shop.sage.co.uk/pdf/connect_for_Sage_50.pdf
This gives an XML import/export facility, this may be enough for my purposes.