So, let me get this straight.
I could go along this trial, get the 3 discs at once option... how long before the trial ends should I send them back? To be safe that they return in time so I can cancel the subscription and therefore go through this whole process free.
Also, if I also wanted to do the £15 amazon voucher offer they have running, would I just create a new account?
Yep, pretty much. I created a new account and it worked fine for me.
As for cancelling, it's best to send all the discs back at least two days before the subscription's set to run out; personally, I'd send them back three days before. But call them up to cancel before you send them back. That way, as soon as they receive the discs, your subscription will be cancelled, and you won't lose any money.