Applies to just about the whole Thompson & Morgan range for on-line orders. I've just used it to order seed potatoes, but could also be useful if you want fruit trees or bushes, or gardening kit. They also have 5 for 4 on most veg and flower seeds and quite a few 99p seeds which might be worth looking at.
TM_TWEB3W
If this doesn't work for you in your shopping basket, use this one.
TM_TN304W
Dangerous stuff for a wet weekend, but could be worth a browse if you are feeling like you have your seedaholic tendencies under some sort of control