AMumInScotland - yes, I agree. A separate UK and English parliaments don't make sense, but neither does legislation being decided by MPs whose constituents it doesn't affect. This is one of the problems of devolution!
The problem is that it doesn't make sense logistically to have a completely different set of people making decisions for England than for the UK, due to the 80-90% overlap. But if you have one group of people making decisions for both, increased devolution to Scotland means that you really can't justify having, say, a Scottish PM, home secretary or chancellor, because most of the decisions they would be making would be for England only. But since they are also making major decisions that affect all of the UK, it's a serious democratic discrepancy that a Scottish-elected politician couldn't have a chance of holding these positions. Devolution doesn't work! That's one of the reasons I'm voting yes.
Some sort of federal UK could work, but only if England was split up into different federated states. Personally, I think this would work well - I'm from the North-West, and would like to see them have more autonomy. But this is a serious long-term prospect - it would be a complete overhaul of the way England works, and there doesn't seem to be much appetite for this in England - so as a solution to Scotland's demands for increased autonomy, it's not feasible. Plus, in a federated state, the key things that are kept centralised are immigration and foreign policy, and these are two issues on which the difference between Scotland and rUK are most pronounced.