Hi Glyn,
Welcome to the forum and welcome to UCET
Cwmorthin is owned (leased) by GoBelow and they can allow you access. As you have surmised, they are very liberal with access and will offer you a key from their offices for a small deposit. It was certainly the case (I am sure it still is) that you need BCA insurance (British Caving Association), which will cost you £20 per year (any caving club will normally do this for you when you join).
There is a fair bit of equipment inside the mine and this is owned by GoBelow and unless you have specific consent, this would be off-limits. Some clubs (including UCET) do have arrangements with GoBelow. You are also quite correct that caving ropework protocols/safety/technique is different to climbing. You are obviously aware of this
We have both weekend trips and Thursday night trips. The evenings tend to be for local explores that allow folk to make it back to the pub in Holywell for a pint before calling it a night. However, weekend trips are all over the UK (this weekend was Yorkshire). We do visit Blaenau Ffestiniog fairly regularly.
All the trips are published on our forum, but the diary is maintained in the Private Members section, and some trips are members only (usually either because we need to limit numbers or because, in the case of ropework, we need to know your capabilities). You would be very welcome to come along on any suitable trip that you fancy. You would be able to see those here;
www.ucet.org.uk/index.php?option=com_kun...&catid=13&Itemid=191
Or
www.facebook.com/groups/30320727365
… Just stick your name down and advise its your first outing and one of the trip leaders will be delighted to welcome you (they are all a great bunch).
Ian