So I dropped on a used SR500 from Andertons . I figured better to spend the same money on a used bass from somewhere with a Returns policy if there was a problem, rather than an ebayer - altough I'm sure it would have been fine. The chap I emailed at Andretons said he'd had a quick play on it to check for fret buzz etc, and it was fine. Arrived via courier yesterday. Apart from a little shop dust it was perfect. I think the packaging must weigh more than the bass - so light!
I haven't done a side-by-side comparison with the Aria yet - the neck feels a little different, but very playable. A great range of tones via the EQ and Pickup controls. I just wish I had a proper rig to play it through. Maybe some time.
ForestPoetry - I will definitely look at the SR305's if I go down the 5-string route - they're mahogany too, although I prefer the "Iron Pewter" finish of the old series to the "Weathered Black" on the new ones. My other preference would be a Jackson Concert Series 5-er.
Thanks to everyone for the advice.