This requires you to make 2 seperate orders to get the series for just £8.70.
BBC shop have each individual series on sale at £3.45.
Add series 1 and 2 to your basket and apply the discount code ARROW21 for £5 off. The order total (inc p+p) comes to £4.35.
For the 2nd order add series 3 and 4 to your basket and follow the same steps.
Total comes to £8.70, a smegging bargain!