I've used this a fair few times and it really seems to make a huge difference. I've never seen it this cheap before, it's normally £5+. Also 10% off on 4 units, so £2.25 each of you buy 4. I've kept stock over winter before and it seems fine.
I usually use the hose attachment that Miracle-Gro make, it's a bit rubbish but it does the job.