Been out with it tonight and as things were cold I got a good feel around... one of the mounting screws for the lower 500 fuel rail had vibrated out and was sucking in air directly from there... hence the hissing.
It wasn't the dump valve at all!
Finger over the hole and the revs dropped... problem solved. Just hope I didn't over do it on a lean engine during my blast around - but all sounds tip top.
Thanks again for the ideas guys
