Hi mumpy, like Mike I was swung by the GRP body on my 2004 Spectrum 419 (same as the GTS 417 bar an axle and a few toys!) Very pleased with mine, only real problem to date was a leaking shower tap/supply hose joint, where you couldn't get to it, but it only dripped via a tell-tale hole into the shower itself so not too much of a problem. Highbrdge, where I bought it, sorted it efficiently on first service and all well now.
What year are you looking at new? If you are looking for the fixed bed model, Michael Jordans in Surrey were doing their GTS Vouge based dealer special "Impression" model for a good discount on 2005 and I think they still had a new 2004 model too.
Andy