Can't cure cistern leak

A year after I fitted it our toilet cistern has developed a drip from the entry point of the bottom inlet.
Typically it's a cheap plastic cistern hidden behind boxing so I have to work with mirror and torch!
It's weeping between nut and cistern. I've fitted a new rubber washer, twice, not over tightened. Still bloody leaks.

 

Is that ok on rubber washer and will it last?

 

Thanks for that. I guess it's silicon then even though it feels like a bodge.
Still puzzled why it's dripping. My only thought is there's a little lug each side of the tank base inside maybe some manufacturing thing and it's very slightly interfering with the washer.

 

I put a new one in just in case the old was dodgy!

 

Well it wouldn't fix with silicon either so I finally had to wrestle the cistern out and ... it's cracked!!
Plastic cistern cracked across from the inlet hole.
It will be a bugger to refit a new one but at least I know the problem.

 

You were nearly right!
Never would have suspected a crack in the cistern body though; it's pretty thick plastic. Wonder if it's stress or manufacturing fault.