For this week, you can book tickets directly on altontowers.com at 2-for-1. Nothing you can't get with a voucher....so far
However booking via altontowers.com gets you into the park 1 hour before the general public, so you can get on some of the more popular rides before the August holiday queues build up.
This may have changed, but last I heard all of these open at 9AM, (that's 1 hour before the general public get in at 10:00AM)
Runaway Mine Train
...so you can repeat ride Oblivion or Nemesis for an hour before the great unwashed masses make it through the gates. And you may even have enough time to get over to Thirteen for when it's queue opens, ahead of the main stampede.
Basically a 2-4-1 with knobs on...if you were going to use a voucher anyway, the ERT is a nice bonus.
And a potential 6% Quidco, I think.