I went to the removal people who last moved me (if you see what I mean). They gave me an enormous, very sturdy, square (this is fairly important) box *and* a roll of tape, and disposed of the crap box I'd got from the supermarket. I love them!
Shame you're not in Chehsire I've got loads of removals boxes from a recent house move looking for homes. I bought mine from an online place when I needed to post a few saddles. Can't remember where though, sorry.