Basically..just the more room the better.
When you have yours out for playtime you'll see what i mean by wider and deeper would be better.
Most cages that you can buy are tall though.
There's not a whole lot you can do unless you make your own.
That john hopewell guy will make custom cages to your specifications too for future reference.
Since you're from the uk check out the chinformative forum. They are all UK peeps and there are a few who know a great deal as they have had and bred chins for a decade or more.
There is also a very very helpful references section with a great number of topics.
Oh and one more thing... For a site to buy wood and chew toys and healthy treats for chins use chinchilla2shop.co.uk and galesgarden is another uk based shop that sells healthy treats
http://www.chinformative.com/The ladies are super friendly...a couple of them travel around the country educating people on chinchilla ownership