No voucher or hidden terms as far as I can see. Just go to your local Slug & Lettuce on a Monday and all the food is half price.
I'm not sure if you can use this in conjunction with the £5 off £10 spend voucher but I can't see why not (if you don't know Slug & Lettuce give you a £5 voucher if you sign up for their newsletter - voucher details attached)
My favourite is the Slug Platter which is normally about £11 so guess this would be £5.50 on Monday. Add a couple of drinks should take it over the £10 mark then use the £5 voucher. Bargain!!!