A little catch I discovered in the current M&S promotion:
Get the £45 shirt for only £21!
Hurry offer ends Monday midnight
For the formal shirts There are 2 offers simultaneously. 2 for £60 and 20% off (for sparks members). I previous such promotions it stacked the offers as first 2 for £60 and then 20% off (resulting in a reduction of £12). Currently it's doing first 20% off and then 2 for £60 resulting in the total being 2 for £42 rather than 2 for £48 (reduction of £18, £9 per shirt).
Same for the 3 for £90 deal.
But hurry, as the 20% off for sparks members ends tomorrow