Many thanks for all the help.
Just to up-date, all is now working OK - I bought another HD - Dell original, brand new, 'pulled' from another Inspiron. The caddy wasn't correct, so exchanged it for my own, popped into place and Lo and Behold! all working as advertised!!
Presumably the BIOS is set up to only work with Dell's own. For the moment I shall just accept that it works and leave the investigation into 'WHY' until later.