eggc-- there is also a bit of leakage to Liverpool. Easyjet offer some destinations like Zurich, Geneva, Madrid which at some times in the past have been on more convenient schedules and/or cheaper than out of Manchester. Forty mins more on the road each way can sometimes be worth it.
The point about the WSP report where all this started is that it makes a fairly good case for the link road from the A65. It will be a controversial scheme but it could yet happen. It's the sort of scheme which Leeds, Bradford and Wakefield districts might all back.