Cheers for that, thankfully didn’t need it.
I’ve got it working! Not 100% sure but I missed something in the BTT docs that might have been the cause. They say to flash the program at 0x8000000 instead of the Klipper default of 0x8008000 (8KiB bootloader) so I guess there is no bootloader on it from BTT.
I flashed canboot setup for an 8KiB bootloader and it just worked right away, didn’t even have to flash Klipper again as it was already flashed at 0x8008000.
Thanks for the clues and the great software and support guys!