Finally found some Thule Bars at a boot sale today for £1.50 so though I'd try out the kit I've been ammassing.
Read a while ago that the 477 kit was unecessary which was annoying as the Thule website suggested it was essential.
Anyway. Findings for anyone who cares is that no, you don't need the short roof kit, but without it the bars are quite close together at about 580mm between bar centres.
Add the 477 Kit in and you can space the rear bar a bit further back making the whole thing a bit more stable (720mm between bars) especially with big things mounted on top. Seeing as 477 kits come up on eBay all the time (think mine was £10 +p&p) I'd say it was worth it.
Pics for reference below.