In reply to dissonance:
You can transfer mid-year - you have to move your existing ISA investment so that you are only actively adding to one product.
Some of the best deals won't allow you to transfer in, so check both this factor and any penalties for early closure on your current ISA before calculating the value of moving.
For the avoidance of doubt, this only applies to this year's ISA to which you are actively adding money. Earlier years' cash can just stay put in the schemes to which it was added if you like.
Mark