Phil,
You're obviously a man who knows how to handle his finances so I'm not even going to comment on that aspect, all I'm going to suggest is to think about it a slightly different way...
How would you feel if you bought the demo car (nicely specced, a great deal at £350 per month) and over the nasty weather, someone crashed into you, or the road conditions were such that you lost control and damaged the car. It would probably take the shine off it and the compromise on the spec and the fact you're paying the higher price would be niggling away in your mind.
Do what I've done...
Got myself a 160k mile 9yr old Fiesta to use while my GTI is on order. The car is due around March time which means over the nasty weather, I'm not too bothered about the car being damaged. It's cheap to insure, cheap to fuel and the whole time you can save the monthly PCP amount as cash in the bank. Then when the car does arrive in the spring and the worst of the weather has gone, you can really enjoy the car. I'm not saying for one moment you wouldn't enjoy it if you got it now, you would, it's a cracking car! But it will be worth the wait!
I'm like you, a few years older (26) but I like nice cars. I had a Mk5 GTI when I was 19 and I can't wait to get my Mk7 but I would rather wait for my ideal spec and a cheaper price, than compromise on the spec and pay more...
Good Luck!