Like a lot of you, I've never managed to really get "into" the later version. But JACKPOT is one of the exceptions for me, so I'm sorry I didn't think of it.
It took me a while to realize which episode of the FIRST version you meant, but now I see what you mean.
I don't know about the later version, but when it comes to the first series, there seems to be only one murder of a mother-in-law. Which is interesting, considering all the traditional jokes about that particular kind of murder!