Not sure of the legal position on this one. I think you are on dodgy ground in that it was advertised at �45 and you decided to place an order, then when you got to till it said �35.
Suppose this happened in a shop. You were expecting to pay �45 but the cashier rang it up at �35 and you paid �35. Then the cashier realises the mistake and says it should be �45. You'd expect to have to pay the difference.
So what next. You could return it and demand your �45 back- but you'd then have to pay �45 to get another!
You could always claim you hadn't noticed the price of the item until you went to the till; you thought �35 was a fair price so you bought it... but because you had to pay an extra �10 your family is having to go without food etc. They might let you have �10 as a customer service gesture.