You should slacken of the the chain adjusters before undoing the rear axle bolt! (That is what you meant by silver plate? Not the one with the notch in it?)
Just had a look on a US fiche and they are quoting around $15 for a replacement plate. Although US and UK don't really compare - it gives you an idea that it's not going to be stupid money!
EDIT - One second reading I don't know which plate you are referring to. Is it
Number 15 on here:
or number 8 on here?
the second part is quoted at $4.