Leaking fuel tap

Having had the fuel tap off for painting tank and replacing filter and o-ring, I refitted it today and put a dribble of fuel in the tank which immediately pissed all over the floor! It seems not to be leaking from the connection to tank but from the tap itself, I think from the front plate (on the tap side). This is rivetted on and I guess is not removable. Anyone experienced this before? The tap has been off the tank for some months and wasn't leaking before. It may be simpler to replace the tap and I have toyed with the idea of using a non vacuum type such as a Pingel, but these are bloody expensive. Any idea if other bikes have a compatible tap?

Now heres the thing! I have a repair kit for my tap which comes with 2 o-rings, one small (tap to tank) and one large which was not in the tap when i dismantled it. There is a groove in one component which surely needs an o-ring but the one supplied is ever so slightly large, not quite a perfect fit. Its also quite thick. Is this the right place for it and do i just need to sqidge it into place and screw it down tight or does it belong somewhere else?

Ok, all back together with o-ring in place.........leak is definately from front plate, rivetted on, no replacement o-ring available!