Netflix Premium Plan for £5.99 at Netflix (new users & 12 months returning users first month only)
To get the Premium Plan for £5.99, all you need to do is select Basic during your sign up. Be careful, though, as if you select Standard Plan you will get Premium for £8.99 and if you select Premium you won't get a discount.
I am not sure how long this sign up offer will remain active, but it also seems users that haven't used Netflix for 12 months are eligible for this as well.
Remember to cancel auto renewal in advance if you don't want to pay £5.99 for the basic plan after your reduced price Premium month. Happy watching!