Cobbling together information received from Mike Lewis-Jones in Australia and Geoff Hughes, the picture seems to be that the studio was opposite what is now The Piazza restaurant, formerly Plaza Cinema. It was upstairs above a shop which Geoff thinks may have been Hampson & James the chemist, 27 High Street (recently the Oggi Oggi bakery & coffee shop).
Eventually the Williams' business was taken over by Pat Williams who continued to use the name Portland Studio (family relation maybe). Pat was a female professional photographer who was mentioned recently on this blog in connection with a Rhyl Golf Club publication c.1950.
Mike and Geoff confirm that subsequently Pat's business was taken over by Bob Jones who is still well remembered by Rhyl residents. Geoff says:
"Bob Jones hailed from Pwllheli or somewhere thereabouts. At first he traded with the name Pat Williams in brackets after his own name. Later Bob moved from High Street to a studio in Bodfor Street adjoining the Wynnstay Hotel (he rented rather than owned the property).
"I vividly remember trooping upstairs to that studio with my parents and three siblings to have a family photograph taken in 1967/8. (I was eleven at the time.)
"Bob later bought premises in Kinmel Street and moved his studio there (the shop is now the Eastern Delight takeaway). Tragically he became terminally ill and the business was taken over by Graeme Thomas who had worked for Bob for some years."