Lockerbie - an "understanding" on PTA

This bit, par 14, of Sir Gus O'Donnell's report is intriguing. He states, in the usual, restrained civil service tone, that the UK government had an "understanding" that the Scottish Executive would accept the Prisoner Transfer Agreement could cover Megrahi if there was movement on airgun legislation and damages on slopping out for prisoners.

Here's the paragraph:

"14. Subsequently it is clear that HMG‟s understanding was that a PTA without any exclusions might be acceptable to the Scottish Government if progress could be made with regards to ongoing discussions relating to liabilities for damages under the Scotland Act for breaches of Human Rights (the „Somerville‟ judgment), and devolution of firearms legislation."

Kenny MacAskill's comments from yesterday make it clear that it is wrong to suggest that he sought a deal on this, or anything else. There was not an understanding but a "mis-understanding".

There is nothing in the papers to show that the Scottish government ever suggested a deal could be made over Prisoner Transfer agreement. Of course, that kind of put your pencils down moment is just not the sort of conversation that would be minuted

But why would the UK government think differently, what made UK Ministers think that the Scottish government was suggestible on this?