XPATH : find ancestor of doctype x for given node?

It's obviously been too long since I queried with XPATH as I'm struggling with this.

I've got a node, I know its ID and I know that one of its ancestors further up the tree is of document type 'en'.

How do I find that that 'en' node?

Thanks in advance.

Matt


This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/64803-xpath-find-ancestor-of-doctype-x-for-given-node