But what happens after the 30th September though?
The bar is to "allow time for a British buyer to raise funds to buy the work instead and keep it in the UK". [wiki]. So, after the date, the minister can issue a new bar or give a license to export. It all depends on if the museum can raise enough money or make a deal with Clarkson.
An interesting case from 2011: a 'Lost Rubens' painting was temporally banned, but later granted an export license. A short, interesting read ( 1page) in the arts council's annual report.
http://www.artscouncil.org.uk/media...rt_Export_of_Objects_of_Cultural_Interest.pdf