You do need to be careful as, they can 'try it on'.
Both my other half and I have done these sort of deals, both with E2Save. The first time she tried to claim the money back, despite following all the T's & C's to the letter, they sent a letter back saying she was too late and it was a different months bill she should have sent.
She wrote them a nasty letter and surprise, surprise, they honoured the claim.
As a result, I sent of a covering letter with my first claim and got it no problem. I'll do the same with the next one in May and hope for the same experience.
If you follow the T's&C's word for word and send off the claim via recorded delivery, you'll be just fine. It's quite nice to get those little cheques through the post every 4 months
Cheers
Paul