Hello Boys Thanks you for the advice .
First thing I did was inspecting the corks as the tank was already empty it was easy to take out the tap and dismantle it.
I do not know the definition of good cork but mine were surely not up to that standard.They came out in pieces and crumbled in my hands.
I think that modern petrol has a destructive effect on them.
There were even pieces in the holes of the tap and they must have been the cause of the spitting back last sunday on the way home with my bike.
I must have been lucky that the bike did not stall completely.
Not having new good or even bad corks on the shelf make me look for an alternative material and I stumbled over a flap of slicon of 3 mm thick, probably a leftover of a dimantled heather or something( never trow things away!)
As far as I know petrol has no effect on silicon.
I made some rounds out of it and even did not forget to make a hole in the middle and after putting it all together I put the tap back in the tank.
After putting some petrol in the tank I noticed no leaks anymore except for some sweating on the reservelever and and the push/pull operated smooter than before.
I do not know if this a permanent solution but my wife and I are happy for the moment and on the next jumble I will try to get my hands on good corks and try out your advices.
Or I will have to start drinking wine as Dave suggested ( for the corks of course), but its a big step from whiskey to wine.
Fingers crossed for my silicon solution......Chris