Thanks for all the replies guys, most helpful and in an ideal world I would go with the Prolong. However as I've probably only got 5 or so miles worth of fuel left it's a no go this time. Car's a daily driver and I need it tomorrow so mail orders also out.
Halfords have the Millers in stock and as it's only a mile or so away and next to a petrol station that's what I'm going to go for. Millers say it adds 2 points to existing octane rating so I assume that if I fill up with normal 97RON super and add the booster then that should put it up to 99RON?
Irrespective, I won't be giving the car any stick until I can get back onto V-power, if they ever get around to re-stocking it.
Thanks,
Neil