I forgot to mention this yesterday... and maybe everyone knows already...
I arranged for my replacement box via instant chat and it was very easy to do. Also, I see that Talktalk have finally implemented the option to save/email/print the chat transcript. This is something i have been asking for for ages... usually after having renewed my contract via chat and then some months later having to complain about not being given what i was promised at that time.
I would highly recommend using chat for upgrades, renewals etc in the future.
At least i have it in writing that it the replacement and delivery is free etc (see attached screenshot of transcript)