Thanks, All.
Ideas appreciated.
Removable bays idea liked, mixture, and likewise RAID1 possibility with the device that you highlighted, but as I only need the enclosure for periodic cloning of the boot HDD and all my other needs are covered, I'll keep it simple (the HDD can stay inside the enclosure until such time as I ever might need it) and low cost.
I may clone to the NAS too, thanks, SD, but for the relatively low cost necessary to incur I'd prefer "plug and play" as my first step to recovery (it is bound to happen at an inconvenient time if it ever does) with the method you suggest as a secondary back-up.
Thanks for your link AAG; from the target page when I clicked it was a further link to what seems to be
just what I want. At the price, I'll take a punt! Cheers.