Hi Stella,
I've had a few bits go astray or get broken when selling on eBay both in the UK and overseas, and the Royal Mail has always responded and refunded promptly within a fortnight of the claim. Mind you there's been nothing worth more than a couple of hundred quid, and the sale and value has been easy to document/prove as I just enclose a copy of the eBay paperwork.
Was it a really expensive item, hence why they might be stalling pending investigation? Where stuff has gone missing on arrival in another country, I've found that RM just refund me straight away as they know it's been nicked at the foreign sorting office and there's no hope of finding it.
But you're right, the complaints procedure is very impersonal and you've got no hope of talking to a sentient being. Where I know the buyer, I just refund their money straight away as I know they're not telling porkies.
Hope the RM get their finger out and either find or refund your package soon.
Theresa