I don't know if a Pit Master IQue will have enough oomph in the fan to do your cooker, if it does then it would be fairly economical coming in a little over £100. You will still have to modify the blower adapter though.
If you were to spend more, you could get a stoker or BBQ Guru which will definitely have a fan option that will work for you. You may still need to do some mods to get the fan to fit. Cost here will be a minimum of £200 but I'd guess more.
Both of these options will need to be shipped in from the States.