Hello, my name is Mark and I will try help you with this.
Much depends on what code they found and what parts they intend to replace.
If a part is needing cleaned then I suspect they are talking about a wheel speed sensor.
I would say they are jumping the gun a bit with giving you a price just now though.
They first need to establish what code is stored.
They then need to check the component involved and the wiring to it.
Depending on the results of the tests they can then work out the cost of repair.
You also have to remember they need to charge for diagnostic time which can be a few hours, depending on at what point they find the fault.
For instance, if they found the sensor was at fault on the first test they did, then £600 would seem excessive.
However, if they found the ecu was at fault after 3 hours of testing then £600 would be too cheap. More likely to be about £1000 to complete the job.
This would consist of say 3-4 hours labour, the cost of the part plus programming and then vat.
A lot of dealers can charge over £100 per hour for labour so costs can escalate quite quickly.