OK either it's me - or this is very misleading...
In actual fact they DON'T deduct the £8 / month you pay for phone XL as advertised - they actually deduct £8 off the monthly cost for the phone plan. So you still pay £8 / month for Phone XL and £7.32 in the case of the Blackberry. Very naughty VM
I had to phone them to confirm this was the case.