Roger Mills wrote:

Can you point me at a suitable replacement for 18" T8's? They seem less
plentiful than 24" tubes.

Google showed me several

https://www.lampshoponline.com/f15w-t8-triphosphor-fluorescent-tube-18inch-15w.html

https://www.edwardes.co.uk/en/categories/15-watt-18-450mm-fluorescent-lamps

https://www.thelightbulb.co.uk/light-bulbs/fluorescent-tubes/t8-26mm/15watt-18-inch

https://www.lamps-on-line.com/fluorescent-tube-f15w-830-18-t8-15w-830.html

Plus numerous Amazon/eBay sellers ...