help!!!!!!hot water tank?

Have a leak in the hot water outlet from hot tank in airing cupboard in my bungalow.
It's leaking from behind the nut against the tank. Have turned off cold feed into tank and drained taps but still seeping water. Don't want to open drain valve in case that starts to leak as well so would then be in the preverbial.
Anything I can use which will cope with sealing it while the joint is wet as a temp measure it ....up the section of chipboard flooring so it's been leaking for some time without noticing. What will the cost be to change the tank if needed?. I'm an O.A.P and don't want to be stitched up with an enormous inflated estimate.