Copied directly from the Royal mail first class FAQ's
For items posted with Royal Mail which have no intrinsic value, or where a customer cannot provide evidence of posting, there will be compensation for loss, damage and delay of a minimum of 6 x 1st class stamps at the first weight step. In the event of a compensation claim for loss we will refund the postage paid or 6 x 1st class stamps at the first weight step, whichever is greater.
For lost or damaged items that have an intrinsic value, with evidence of posting with Royal Mail (such as a certificate of posting) and evidence of value (such as an invoice or receipt) customers will be entitled to a postage refund plus compensation for actual loss up to the value of the item, or 100 x 1st class stamps at the first weight step, whichever is the lowest;
In short, if you have a receipt to prove postage you should get