Today, inspiration hit me and I started to look at it a bit, for now first with gattool only.
Took a while to figure it out, BLE is a bit of a mystery with UUIDs and handles and so on.
Notification handle = 0x0012 value: 03 c8 00 00 00 06 99 e1 00 45 99
Byte 1: What data is available, crank=1 and wheel=2
Bytes: 2,3,4,5: Wheel revolutions
Bytes: 6,7: Wheel event timestamp
Bytes: 8,9: Crank revolutions
Bytes: 10,11: Crank event timestamp