Cheers for the suggestion!
So what I did (I couldn't be @rsed to undo all the heatshrink and cable ties again) is downgrade to CF1.12 as I couldn't get this to work with 1.13. Then tried both passthrough options in BLHeli suite and it worked finally with the AMTEL one! It says they are SN20A with BLHeli bootloader so would I be right in thinking someone has changed the bootloader? (2nd hand drone)
Anyway it worked!! And now on 14.7 which does seem smoother to me (was on 13.1 before!!)
The moral of this story is to persevere even when you just want to through the thing out of the window