Advice?

No you dont need a new one and it dosnt matter how tight you do it. Try cleaning around the base where the water goes down and out to the pan. Make sure there is no grit, if the rubber sill dosn't sit right water will still trickel out.

Hope this help.

Hi Steve, I can only tell you what I'd do.

There are only two possibilities. 

Water might be getting through where the mechanism is fastened down to the WC tank. It's a bit unlikely, but if the seal is in any way damaged, then it might be worth drying out the tank and smearing some silicone sealant around the outside of the seal.

The most likely is as Darren has said. Try applying slight finger pressure to the plunger, and see if that cures it. If it does, you've the choice of removing it and giving it a good clean, or increasing the weight of the plunger by wrapping something like solder or copper wire round it. Also check that the lift and drop part of the mechanism isn't sticking, and that it's allowing the plunger to fall freely.

Great advice fellas thanks

Similar advice to the above, and something I encountered too when changing the syphon in my home over the weekend!

I was getting continual dripping from the water feed pipe, even though (I thought) the float had been returning to its correct position to close the valve with the plunger mechanism. Numerous removal and cleaning of the rubber seating in the valve and plunger didn’t make any difference - found out that the ball valve float was not lifting to the closed position due to it slightly catching on the inside surface of the cistern tank - a few modifications to reduce the length of the float arm to ensure full free flow , problem solved, no more dripping!