Natural Resources Wales which have been looking at blow out risks at disused mines across Wales. I first became aware of this study when Cambrian Mines Trust, Cadw and other stakeholders attended a meeting at Cwmystwth around a year ago as that mine site was, it seems, higher up their priority list than some of the Gwydyr mines which they have now reached.
I concur that Parc No.3 Level presents a potential significant risk should the drainage through unknown rubble filled voids in the base of Kneebone's Cutting to No.4 Level become blocked. In this instance there is a straightforward solution - to pipejack a tube (or another technique) through the fall a short way into No.3 Level. Roy: maybe Cave Access Ltd, as a major stakeholder, ought to make representations? PM me if necessary.
Back on topic, I'm afraid I have little direct knowledge of Coed Mawr Pool but hopefully the above notes put the enquiry into the context of the wider risk assessment study.