Just spoke to the technical dept on Vokera's customer helpline and
they've confirmed that i need a new o ring (part number 0716). Curzon
(who do on-line sales at http://www.curzoncare.co.uk) has an outlet
close by so i'll pick one up tomorrow morning before work.

Engineer on the phone said that if i remove the two bolts which screw
towards the centre, beside the part he called the manifold (the disc
shape thing), it should separate from the manifold and i should see the
o ring. Apparently this pipe/valve i am loosening is only meant to
have two bolts in it, even although it has threaded holes for four.

Cheers for your advice & tips Paul about replacing the o ring. By the
sounds of it, i won't be able to get it finished without help - however
will give it a go, and make sure i have hi-temp sealant at the ready.

Probably give it a go tomorrow night.

cheers

Daljit