Re: 20year old with a gtv or 156
23 years old, 156, 25,000 miles annual limit, 4 years NCB = £420 (£250 excess).
Alternatively, Confused.com is a good place to start. If you find a good quote, go to the insurance company's own website and run through the quote again; it usually works out cheaper still.
At 21, just before I bought my 156 I shopped around for quotes first. I found some for £800-£1000 for a 2.0.
Don't forget that your postcode has a big impact.