may be a silly question but i will ask it

yet again I have ordered an item which is smaller than expected according to the measurements, if I was to call cs and ask for a larger size will they still charge me P&P for the replacement.

As far as I know yes, they will charge the pnp.

It's worth phoning them or email them direct (not using the contact form on their website) and stressing that the quoted measurements don't correspond with the garment's actual measurements. This may at least illicit a prepaid return label, particularly if it sounds to them as though you won't re-order if you'll lose the first lot of postage and the cost of the return.
The secret with customer services is to lead them to the remedy that suits you. Unlike most mail-order companies they don't seem to be able to consistently process a return that requests a replacement; mostly they automatically refund and then you find the item you wanted has sold out. I hope you get it sorted.

I personally have never had a problem with customer service to be honest. They've always been helpful when I've explained something that has annoyed me, I'd give them a ring.

I think it might be considered a new order therefore you would have to pay p&p, but as I said they can also waive that.

Whenever, in the past, I sent something back for an exchange, no matter how clear I made it on the returns slip, no matter how clear I made it on the phone telling the CS advisor it's used or broken or broken and used, I'd always get a refund, always. Probably they think if you want it badly enough you'd re-order paying more p&p, I never did.

the fact the measurements are wrong means you will be allowed a free return with the pre paid label.

Yes allowed for free.