It appears not everyone knows this so thought it was worth a new thread (think I picked it up from moneysavingexpert):
You don't need to spend (eg) £90 to use the '£9 off a £90 shop' code.
Book a slot, fill your basket with £90 of stuff (I do 10 big boxes of washing powder) and check out with the code.
Then log back in and do 'Change My Order', delete all the washing powder, do your normal shopping and hit 'Confirm Changes'.
Even if the new total is less than £90 the code is still applied when they bill you.
Obv this works for any 'spend more than £x' code - just make sure the initial shop is more than the threshold.