Perhaps it was heard once at a pub but Adge could not remember the details.

Yes, Keith, of course you are right - the lyrics themselves acknowledge the influence of "My Old Man's A Dustman".

But as to your comment I quoted above, I don't think the lyrics of Adge's song indicate either way. He could have known the lyrics of "Dustman" off by heart and still written that - it's a different song so there is no attempt to parody the original.