OK, as a side point, I added bbc.co.uk using the web front end and it created the regex for it... however, I don't think this is the problem.
I've just added 192.168.1.109 (harvs-iphone) to the "default" group and browsed to bbc.co.uk, as you can see from the log, this is still unblocked (as expected).
2021-06-23 09:03:26 HTTPS idcta.api.bbc.co.uk Harvs-iPhone.broadband OK (forwarded to resolver2.opendns.com#53)
Now, I've added the bbc.co.uk domain to the "default" group and browsed again, this time you can see that it does block it (as expected)
2021-06-23 09:07:05 HTTPS www.bbc.co.uk Harvs-iPhone.broadband Blocked (regex blacklist)
Finally to confirm I've removed 192.168.1.109 from the "default" group and it continues to block the domain (not expected):
2021-06-23 09:09:28 A a1.api.bbc.co.uk Harvs-iPhone.broadband Blocked (regex blacklist)
It's leading me to think that pi-hole isn't recognising the device / the groups it is in, despite it appearing in the logs correctly and appearing in the clients list. Any tips to debug further are greatly appreciated!
Thanks