Main dealer repairs and parts costs

I would like to highlight the potentially incredibly high costs associated with dealing with a franchised dealer for motor repair work and especially parts.
My son is an electrician and in 2014 bought a new Vauxhall Combi 1.3 diesel van for his work. He has always used Vauxhall main dealers for servicing. In recent months the van began to run very roughly and lacked power. 2 weeks ago He took it to SLM Vauxhall in Tunbridge Wells. They said it needed new fuel ejectors. These were replaced at a cost of a little over £2,000. My son was told the EGR valve was also cleaned. On collection on the Saturday my son found the van still ran very roughly and lacked power. The next Monday he was told that a new EGR valve was required and was quoted £1316 plus VAT for the valve with a total cost to supply and fit of around £2,000. I contacted a small local garage, Sovereign Motors in Tonbridge who quoted £195 +VAT for the valve with a total price to supply and fit of £522 including VAT. A Pierburg brand valve was fitted which is the same brand as the original valve that was replaced. The van now runs perfectly. How can a main dealer justify such a phenomenal mark-up on the part (£1316 compared to £199) The local garage obviously made a profit on the part.
People like my son go to main dealers because they believe there is added security. In fact they may be being ripped-off.