Graigfawr,
Thanks for posting up the link to the video, for someone like myself, who has only read about the Halkyn system, it makes for interesting viewing.
A number of points come to mind after watching it.
The first is this group, had carried this exploration previously, but presumably this was the first time they had admitted to,videoing it?
United utilities are certainly tempting their luck with not keeping an eye on the condition of the Tunnel.
If it becomes more blocked, than it is, then there is the distinct possibility of some sort of water-blow out. This almost took place at Leadhills, when Gripps level was blocked.
It occurred in Derbyshire when Magpie sough was blocked, and may have happened again, with the potential for even larger damage except that here P.D.M.S, arranged to,clear it out.
In my opinion any ‘official connection with the Utility company should mention this possibility.