Hi, this has just happened to me on my very old Triton Opal. I rang Triton Customer Services on 024 7637 2222 and they explained it was the "pressure release device" and that a ball bearing had moved to force water through the overflow. You can't move the ball bearing back, but they are sending me a new part for £8.75. They say it is very easy to fit.
Peter Harding, September 2006