Hi, Mark here and I will try help you with this.
Most likely cause is a faulty wheel speed sensor. When the garage checked it and they 'couldn't get any on the diagnostic', does that mean there were no fault codes stored or that their machine could not connect to the vehicle. If it means there were no fault codes then what should have been done was a road test of the vehicle with the computer connected so that the live data from the sensors could be observed at the same time. You could then see if one was displaying different information from the others.
If it means their machine could not connect to the vehicle then either their machine is not up to the job or there is a fault on the vehicle with its internal computer network.
The difficult part for you is the expense. Sometimes easy faults to repair can take a long time to find making them expensive or faults that are found relatively quick require expensive parts to repair it.
I hope this gives some help on how you continue.