Thanks for the replies guys and I've done the checks suggested and everything is OK, no bubbling of water, also checked the oil but there's no sign of scum/emultion. The steam from the exhuast must have been just a lot of condensation from it being stood a long time because it's stopped, there's vapour on start up but it goes after 10 minutes. I think it must be an intermittent leak but I had it running for over 1/2 hour on the drive and nothing. I'm going to run it and check the water daily. I read a thread about an O ring at the back of the plenum but it looks like a pain to get to.I guess it's down the garage if it continues.