Just a little warning on the 3 for 2 offer. If you are taking 1 or 2 of the items back for a straight swap, make sure you take all three items back.
M&S systems can't cope with a straight swap in this case. You will get a refund for just 2/3rds of the price but then get charged the full price for the replacement.
Ie I bout two pairs of trousers at £14 each and a pair of shorts for £8. I needed to replace the trousers, they refund went through at £10.80 but the replacement were £14. Thankfully I had the shorts with me and the way round it was to refund the shorts as well and bought them again so I was covered under the 3 for 2 offer again.