I'm taking advantage of the T-Mobile offer of 6 months free internet when you buy a new smartphone on PAYG from them.

Have any of you done this?

I'm worried about it because I'm planning on keeping my old sim card. If I buy in the shop will I be able to have the offer registered to my old sim card?

I presume the offer is linked to the sim card in the box by default....?

I don't understand why having a new Sim card is an issue?

Sorry for not being clear- I meant I want to keep my old number.

Will the bods in the T-Mobile store sort this out.....for free?

Gary-the free 6 months is offered when you activate the SIM that's in the box with the 'phone. Do that first by making a chargeable call or text-you should then get a text from Tmobile offering the free internet: reply to accept it.
THEN arrange porting over your number: I assume it's from another network and you have a PAC code?
Do this by dialling 150 and follow the prompts about "bringing your number from another network" and customer services will arrange the number prting-it should take less than 24 hours.
Oddly-and I don't know why-Tmobile can't swap an EXISTING Tmobile prepay number onto the SIM in the box.

Thanks Bagsbunny.

I am already with T-Mobile, so it looks as though I'll have a new number.

You can always port out to another network then port back in to t-mobile.