...I was just wondering if this was a common thing to do? Or do people normally go for aftermarket stuff instead?

And are the offsets etc the same between 16vt and 20vt making stuff interchangable?

Lastly what's the difference between the two types of 20vt 16" alloy, and how can I identify them?

I did a search but didn't get the info I'm after, sorry for the slightly daft questions! :-)

20vt wheels will go straight on a 16vt. The later 20vt wheels were fitted to the Plus model and the 20vt 6 speed. They have a more complicated design.

Cheers! :-)