Is there a definite difference in meaning?
Or does it depend on the context?

It seems to me that le chéile means together and lena chéile means with each other, or have I got it trína chéile?

My understanding woukd be that

le chéile means "together" (with the other)
and
lena chéile would be more like "with each other".

Fairly similar really, although the second is a bit "closer" in nature.

Thanks for confirming my own feeling about it.

I noticed that someone had posted on another forum a sentence with lena chéile and a little later had changed it to le chéile, I'd just never really thought about a difference before.