I am not into ponds' exactly..
But we do have many members' who are..
and I am sure someone can/will come thru and
lend you a tip or two on your question..
In the meantime while you're waiting a bit...
There's another poster in your country who is in
the process of building a pond..
Take a peak thru his post and I will PM him and see if he is still available to maybe make a comment or hint about what he's done and that may help you in your pond adventure... My watergarden im currently building at present by David Mack
in Kilkenny rep of Ireland