The evidence based success of the 5p levy is shown by sales of bags in shops. Less bags sold = less bags sent to landfill or littered. Liek you said though that is pretty scanty evidence at best.
What I don't like is that the law was brought in for Ireland and Wales and has been hailed as a success based purely on the amount of plastic bags bought at supermarkets. While that makes great headline figures I haven't seen any figures on comparitive sales for other products like bin liners and pet waste bags to see if those have risen sharply which you would think would be crucial to the experiment. Whilst I'm all for reusable bags I think this should have been dealt with in a more thorough manner.
You live in South Wales, have you not noticed less bags in the the countryside/streams/rivers? I certainly have. What more 'thorough manner' would you suggest?