Both do similar work, but both charge way too much.
Regal did a 4x4 Nova for something like £10,000. Basically took the bits from a Calibra and transplanted them in. You have to wonder how hard this is and how they can charge so much.
To be honest, the only difference between Courtenay and Regal, and the regular motor tuners are they specialise in Vauxhalls, and they charge three times the amount.
I wouldn't use either.
"C20 LET bang"