Hi Stewart
If I were you, I would get them to check whether the power is getting to components (using a multimeter), for example:
Is current getting to the starter motor solenoid terminals?
is the starter solenoid engaging?
do the headlights dim when turning the key?
is the battery 100% condition?
are the chassis earth terminals secure and free from corrosion?
are the battery terminals tights?
There are so many fundamental tests to go through before condemning components and always start at the source of the power supply - the battery!
Let us know how these checks go
Cheers . Trevor