Displayed at £29.99 but comes up as £19.95 at the till, to get it without having to purchase a top-up you need to tell them you are already on O2 PAYG and want to keep your number so can you have it as an upgrade. I have done this for my last 3 phones from CPW and never had to buy the top up.
You will need a PAYG sim for this deal as they ask for your number, I don't know if this will work for all networks I've only tried with O2. This is instore only, the link I put does show the price as £19.95; but when you click through to the next page it's £29.99 and requires a top-up if bought online.
Receipt to follow below: