Goto the conversation list : http://www.acorndomains.co.uk/conversations/
Tick the tick box next to one of the conversations, a popup will show. To the right of that popup there is "select all" which will then select all of those messages on that page, you can then goto page two, do the same again and once you've ticked all the pages you want press Leave conversation.
Thanks - yes did that now - didn't take as long as I thought it would.