I've just received a reply from Interparcel saying they'll pay me the £50 maximum for the claim (that was my fault for not selecting the enhanced insurance). Plus, they're refunding me the cost of the delivery booking too.
That's reassuring
The other really upsetting thing (but which is entirely my own fault) is that I hadn't checked the "enhanced insurance" option when I confirmed my courier booking.
So, I'll only get £50 as compensation!! Aaaaagh!!!