Testbook is the Land Rover diagnostic computer system, which plugs in and reads the fault codes off a vehicle. A fault code is stored when a vehicle ECU detects a fault on its system, and should pick up an intermittent problem of the type which makes a warning light come on and then disappear even if the vehicle currently seems OK.
Testbook/T4/IDS are the main franchised dealer systems, a good independent garage may have Autologic/Rovacom system which will do the same and be cheaper in labour
The chances are you will need a new wheel speed sensor fitted, which is not an uncommon problem on any ABS system, but the fault codes in the vehicle ECU should tell the mechanic what is wrong. Anybody who says they can fix it without a computer is just guessing! They might get it right but it might take a few expensive goes first...