Ok… more progress…
Reading further and as the setting isn’t available per extension in FreePBX I thought I’d look into setting it at a global level in sip.conf.
By adding it through the FreePBX GUI via “Asterisk SIP Settings” under tools at the bottom I added: subscribecontext=ext-local.
This forces Asterisk to use ext-local as location of hints for all the BLF functions which allows extensions to use BLF even if they are in a custom context with rules defined.
Hope this is of help to someone.