In reply to ByEek:
> I think you have answered your own question...£50 for a tarp seems a bit dear to me
Yes, I think I'm down to just deciding between basic tarp and one of those Oxford bike covers richlan refers to. I didn't mean to imply I'd found a tarp for £50 - I'm sure they are cheaper than that, possibly just the way I worded the OP!
Thanks for the help everyone!