I bought from these guys 2 weeks ago!!
The +33.35 plate is METAL, the one that is an extra £6 with the box is PLASTIC. That's the difference. My bf is going with the metal one (we bought both in the end because I wasn't sure what I was doing). I think you can buy the rack with no plate though if money is a worry

The plastic one will do the job I am sure.
Motorbikes and parts gave me good service though it took about a week for them to get the rack, I'll be buying another rack and box for myself soon from them as well.