I spoke to my best man who said it's pretty random - he's head of one of the digital departments at Barclay's in Canary Wharf, Barclays pay his (large) salary and they wouldn't even give him a mortgage!
He recommended I tweak a couple of things, which I tried, but rejected again.
Have bought it outright - £1800 bit of a hit on the wallet, but looking forward to it!
Can I ask what the tweaks were? Both me and my wife were turned down with excellent scores and good credit history