Just to add to my previous post, I had been thinking about buying new bathroom towels recently, and a couple of weeks ago got an offer of 20% off towels among other things, so I took advantage of it! I also bought two sets of Egyptian cotton bedding back in January with a 20% discount after buying a new bed. If I see something I fancy in Marks, I sometimes wait (not too long, they sell out!) to see if an offer comes through for that category, and it often does. Usually offers arrive every week on a Thursday afternoon, so I watch out for them.
And don't forget, your chosen charity benefits every time you use your card, which is a nice bonus.