M&S have a glitch on their Christmas hampers where it is taking the 25% discount off twice. Plus you can use the code XMASFL15 to get another 15% off. So the cheapest hamper you can get is the Sweet Treats at £8.75 but there are some bigger savings to be had on the larger hampers.
For example, the Festive Indulgence one comes down from £150 to £52.50!
Spotted over on MSE (thanks to OP)