Ah, I tried that, and have just been released on bail...
Sorry to bump the thread but after being told about a dozen times by numpty Royal Mail call centre staff that Royal Mail 'absolutely' won't pay out a penny and Australia Post being completely unhelpful and failing to take any responsibility whatsoever, I tried enquiring at my local sorting office.
The chap on the desk gave me his manager's number. One phone call to him and I was asked to fill in a 'P58' form and drop it in for his attention, I'm now due to be paid £36 compensation which isn't the full cost of the items stolen but is better than the AU$50 maximum (about £24) that Australia pay out as compensation, not that Australia Post seemed to want to have anything to do with it.
I never did hear back from the 'Investigation Team Manager' despite leaving voicemail and email messages, Royal Mail are obviously quite happy not to respond and/or investigate knowing they almost certainly (I can't prove it, but let's face it my stuff was stolen and replaced with 3 skanky pairs of UK-labelled boxer shorts after the package had gone through customs) have criminals stealing mail.
So my advice is don't phone Royal Mail customer services - they're generally worse than useless. If your post gets tampered with and you want something done, go in person to the sorting office and get a human being to deal with the enquiry.
Ala Carte Ti, Mantra, Attitude, ancient steel Trek