Hey Michael,
Believe it can be done via the API but we've not needed to look at that yet.
Yes, you can do it from the web API, using the advanced search or activity functions.
1. Can it be done via the web site (are we being thick?!) ?
Not yet. In about 1 and a half weeks you will be able to (we put a significant amount of time improving this in LimeLM, and it will be released shortly).
2. If it's "easy" to do via the API does anyone have some code they could send us to do it? Ideally .NET winforms but if it's some scripting language I'm sure we could get to grips with it...