Rather late with a reply for this, but here goes.
I've never seen any reference to a James Owen Farrell as a mineral water manufacturer in Llandudno, so I can only conjecture that he was not in business for himself, but rather an employee at one of the other factories, probably that of William Hill.
Salford Villa was the home of William Hill (Hill was born in Salford, hence the name). The factory was attached to the house. Hill died in 1928, but by then he was retired and living in Deganwy Avenue, so evidently Slack & Cox had taken over the business. Judging by the Slack & Cox bottles found, the takeover must have happened around the time of the Great War.
That Slack & Cox sign is a beauty. If it ever became available I would love to add it to my collection!!