A few of these are a penny cheaper at Amazon, but Quidco should mean that ordering from HMV offers the better value.
For those looking for the other seasons:
Seasons 1 to 3 (in one set!) - £7.89 delivered from Sendit
Seasons 5 to 6 (Gift set - with mini 'Puffy Shirt' and a limited edition script) - £9.78 from Amazon
That means you could buy the complete series for under £58!