I used Special Delivery before Christmas. It was 3 days late. I got full postage plus �5.00 extra compensation back.
It was the only time I've ever had aproblem.
It's like any compensation claim - once it has been decided that they are at fault then it will be down you you to show how much by. The cost of postage is easy because it's on your SD Slip, but the legal onus is on you to prove how much the contents cost. I don't think they'd quibble too much if you had receipts, as to whether or not you'd put the items in (how would anyone prove without a full checklist signed by whoever served you etc) unless the amount seemed excessive.
Put yourself in their shoes - would you pay out without some sort of proof?