Quick update. I emailed the contact Barry & Di supplied and received a very quick reply giving comprehensive information along with a photograph of a finished installation.
I have been looking at options to achieve this end result over the last year, but I didn't find any "delay timers" I thought were suitable ... and some were pretty expensive. These are very reasonably priced as is the fitting, and some sensible thought has been given to this.. We are going ahead with getting them fitted.