The OP is happy to give an update on the progress made.
Decided to settle on a 'flexible' scale of between 1:19 and 1:24, with choices of rolling stock being based on mainly the width of items available and whether it all looks correct together. (If anyone asks, the gauge of the fictional prototype is 2ft 3in, with no measuring tools permitted near the garden!)
However visible progress has been mainly in the construction department. I decided the railway needed to be a useful 18 inches off the ground adjacent to the potting shed that will ultimately house the loco shed and sidings at one end of the line, and the terminal station at the other. The line will meander around the edge of the garden, at a gradient of about 1:45 to maintain a reasonable operating height. To date, about 20ft of timber 'table' and half a rockery have been constructed, with output depending on the weekend weather forecast and completion of some professional coursework and exams recently. However my thoughts are turning to track (which I intend to build myself) and rolling stock again. Of the latter, AnDel Models seem to be producing some very reasonably priced, simple (and narrow!) rolling stock which is worth a look. As my ultimate maximum width of stock is likely to be in the 125mm region, I suspect I will be making several visits to their website over the next couple of years.
I also have some very solid ideas about signalling with block bells, but perhaps more in the future about that! Don't wish to get ahead of myself. I am very surprised about what is being achieved with little outlay; the amount of free material available for use in construction is much greater if you ask around...