East Anglia.
DP is late 50s, certified motor mechanic, RAF trained, worked in the industry all his life.
In the last few years he has had a number of jobs, in house mechanic for a small haulage firm, mechanic at a small bike garage, MOT tester at a small garage.
Salaries for all of these have been around £30-32k.
He is now working for a larger (but local not dealership) business doing a couple of specialist roles, started on £35 last summer and had a payrise after completing his 3m probation to just under £40k.
I also do the accounts for a couple of garages, again paying around £30-£32k.
I'm surprised by your son's salary.
Edit: having just looked at indeed I seem that my "ideas" are probably a couple of years out of date - as are DPs previous salaries and my client knowledge.
Local MOT jobs are advertised around the £30-34k mark, and as MOTs are generally loss leaders I'd expect fully qualified mechanics to be on a bit more.