ZP seem to be a bit hit and miss with their refund procedures. It seems from past comments on here that they will always quickly refund for dented 'tins' and obviously damaged goods, but certain types of damage seem to be beyond their capabilities to figure out.
I once had a delivery where the an OKO 40L bag was so torn (box wasn't damaged) that I couldn't get it out without it spilling everywhere and had to scoop it out into plastic bags, losing some on the way. When I complained, they just said they would talk to their packing department! Okay, I didn't lose much litter, but I should not have been sent a split bag, and have had to repackage it myself before I could move it. A gesture of good will discount code or something would have instantly made me get over my annoyance though.
The problem is that they are the cheapest, and have products that are very difficult to get anywhere else so they're not really going to lose many customers over this sort of thing.
ZP are huge, but it does surprise me how much money and product they are prepared to waste for the sake of a bit more thought when packing. Not really enviromentally friendly either.
I think in the OP's circumstances I would refuse the offer of £2 and try to escalate the complaint to someone more senior to deal with it.