Now, I'm not convinced it was mechanically sound - the steering was heavier that the two petrol ones I've driven and there were a lot of vibes coming through the wheel. Also, when I got the bite on the clutch, there was a loudish squeek from the engine bay, presumably the gearbox. Should there be much difference in steering feel between a 2007/08 diesel and a 2010 petrol?
The steering might be a bit different between a petrol and a diesel, but probably not much. That said, it is probably more down to tyres (type, size, pressures) and alignment than anything very serious, but it is difficult to say.
The 'vibes' coming through the steering; if it was during braking, then it was probably just brake discs, at one particular speed, might have been been wheels out of balance (and maybe that points to someone having played silly buggers with the wheels, for some reason - refurb? moving more worn wheels around?). Any other cause sounds like trouble.
The clutch thing sounds as if it might be more serious, though; maybe first signs of a DMF problem, maybe lubrication of the mechanism?
The big question, I think is whether the supplying garage are committed to fixing any problems, at no cost to you; if they are, and you trust them, then it isn't such a big problem, if they aren't, then it is a problem and, if there is any uncertainty, walk away.
My last car was an Astra VXR so everything feels mundane now which is half of the problem.
Well, yes, but if you can't get used to the lower excitement level, buy something else. It isn't going to get better. You could get a 200 hp diesel (slightly more with a bit of tuning), but it isn't going to be a VXR. It can be an excellent mile muncher, but that isn't going to be as immediate as a car like the VXR.