Welcome to the forum. Cobgratulations on your new to you ST1100.
The crank angle sensor is a part of the ABS unit, I believe. There are four wires going to each ABS unit. 2 are for the motor, 2 are for the crank angle sensor.
The front ABS modulator is somewhere under the fairng to the right of the headlight. Roughly the yellow shaded component. The red shaded component is where the crank angle sensor is connected to the loom.
If the ECU isn't getting data from the crank angle sensor, it may be because the fuse to the motor has blown. When you first turn on the ignition, both ABS units whirr briefly to make sure that the ABS modulator is in the correct position (ie ready to release the pressure in the brake line, not increase it !). You can put your hand on the ABS unit to feel whether or not it is working when you turn on the ignition.
I have had blown ABS fuses simply by having too much fluid in the reservoir.
It would help enormously if you could let us know exactly which model you have. The white label stuck on the frame under the seat will tell you. Eg ST1100AY - or something like that. There are some big changes related to ABS and brakes over the years. My only experience is with the ABS on the 2000 ABS model.
Whereabouts in North Yorkshire are you ?