I had been looking to order this last week but didn't have the time. I decided to order it today and the price has dropped from £9.99 to the cheapest price at £4.89. I'm glad I waited. I checked a few other sites too (see below) and the prices are reasonable.
I think this is a bargain. I ordered Season 4 two weeks ago for £9.99. Season 5 is supposed to be the "best" series.
You can get both Seasons 5 and 6 from Amazon.co.uk for £9.99 but I prefer to order individually just in case I need to sell them in the future (saves on postage costs). Quidco can be used too.
I ordered from The Hut because I had a voucher code. I have put a number of sites below because some people may have discount codes.
Cheapest is £4.89: Sendit
£4.93 The Hut
£5.77 PC World
Each DVD set has 4 DVDs.