htaccess goes round in circles and so do I

My blog will not load (the dashboard is OK, however.)
Firefox says:
> The page isn’t redirecting properly
> Firefox has detected that the server is redirecting the request for this address in a way that will never complete.

(IE is similar).
The problem is resolved if I edit out one line in my overall root htaccess.
The WP htaccess in /blog reads:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]
</IfModule>

# END WordPress
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
That overall root htaccess reads:
~~~~~~~~~~~~~~~~~~~~~~~~~
# redirect non-WWW to WWW
RewriteEngine on
RewriteCond %{HTTP_HOST} ^themagicofyou\.co\.uk$ [NC]
RewriteRule ^(.*)$ https://www.themagicofyou.co.uk/$1 [R=301,L]

# steers 301s to the index page
redirectMatch 301 ^(.*)/$ https://www.themagicofyou.co.uk$1/index.php

(Plus there are dozens of explicit “redirect permanents”.)
~~~~~~~~~~~~~~~~~~~~~~~~~~
The line which causes the problem is this one:
# steers 301s to the index page
redirectMatch 301 ^(.*)/$ https://www.themagicofyou.co.uk$1/index.php

If I comment this out, the blog loads fine. I have 301’s coming in from an old site, so I do need to steer 301s to the index page.

How do I make the htaccesses play nicely together?

Many thanks

Andy