This was more interesting than we thought it would be!
We had to change 12 year old links to index.htm to index.php! Not a case of simply adding:
redirect 301 /index.htm https://www.spacecake.co.uk/index.php to the .htaccess file – oh no!
After a good deal of head scratching we came up with this: RewriteRule ^index\.html?$ / [NC,R,L]
All is working well & the old homepage has retired gracefully.