There was a leak coming from the back of my loo where the tank fills the loo. I found out that the bit that comes straight down fro mthe tank and goes intot he top of hte loo when flushed was leaking.
I figured out that it was a rubber flange thingy (technical name) and managed to get a replacement from a local plumbing merchant. I asked if any selant was needed and he said no.
I have just changed it over, and thought that it was sorted, but unfortunately, there is still a very small leak every time you flush - literally only 3 drops.
Sealant shouldn't be needed as long as the new rubber seal is a tight fit. Try taking it off again and repositioning it.
Also check that the water is not running down from where the pipe is connected to the cistern and running down to the bowl as this would make one think that the seal is leaking.
If this doesn't help use some silicone sealant when you reassemble it and that should do the job.