I ran across this term and the concept on the EPSRC site with the title What is a Sandpit? and it is intriguing to me. Googling turned up the following (along with a lot of US references to toys and environmental issue related to sandpits):
A BBC article from a writer who attended a session on bridging the Digital divide
This post is from the Warwick IMRC
With or without the driver of a funding being made available at the end of the session, this looks to be a significant process improvement over either typical brainstorming/problem solving sessions or paper based grant submission requests. The winners of funding have a much better sense of the interdependencies they face and the losers have gained something (perhaps quite a bit in terms of both contacts and knowledge) for their time.
Wonder who in the US is giving this approach a try? Still looking around on that topic.