If you look into it a bit deeper, you will find the £300ish deal is probably for a 5 door manual, with a 5-6K allowance.
More allowance, DSG, 3 Door, metallic paint etc can all bump the monthlies up considerably.
The mega deals of 12 months ago are long gone.
I looked into it myself last year when I considered moving the ED30 on, but when it came down to it, I wasn't prepared to pay £17Kish over 3 years for the spec I wanted. It didn't make financial sense when the Eddy owes me nothing. A perfectly decent, but older GTI with just fuel as the monthly outgoing, or £400 a month + 3K down for a car I know for a fact I would get bored of after 6 months, and I can't modify it.
Have you test driven an R yet to decide if it's worth the debt?