A £47 promotion is news to me. (Just seen the post saying it's an MSE promotion) PP were offering £50 to referrer and referred until yesterday, so it will be back to £25 each today.
There are posts on these pages about the pros and cons of the referral system, but I think you have raised a new point. Their own referral reward system rewards customers who recommend Pure Planet to their friends, so there's a reward for loyalty there. And then the longer you have been with Pure Planet, the better you feel about all the money you've saved - loyalty is its own reward in this case.