The following is taken from an interview Steph Perry from Rock Notes On-Line did with Steve Hogarth from Marillion :
We heard that you performed with Dream Theater at the Ronnie Scott's show?
They said they were doing a little gig, and the idea of it was that they wanted to perform songs that had influenced them in their early years. And to collaborate with artists that continue to influence them. I know that the drummer, Mike Portnoy, is a bit of a fan of Marillion and he invited us to perform his favorite song, which was Easter from our Seasons End album. So [Marillion guitarist] Steve Rothery and I performed Easter with Dream Theater. I also did [The Beatles] Happiness Is A Warm Gun, alternating verses with [Dream Theater lead singer] James LaBrie. Mike emailed me actually just this morning and said he's putting together a video for Dream Theater and wanted to include our performance from Ronnie Scott's and did I mind. I told him, of course I didn't mind!