My mum had a carton of apple juice once full of wood lice, the woman she spoke to said to throw the woodlice away and just return the packaging, but then the company said that because she hadn't returned the bugs as well, there was no proof that the bugs were in the juice at all, so she had no claim against them.
Keep a picture and details for yourself too - if you're not happy with whatever Waitrose offer (though they have a good reputation) get in touch with your local Environmental Health Office.
I have only ever seen those shield bugs (and I didn't know that was what they were called!) in high summer - never in January, so I hope it's unlikely they will try and suggest it came from your garden.
All I can say is that I know of their food research dept up at Bracknell and they WILL look in to it , thery are very thorough and will take it seriously- you may even get a fulsome bug report back. Unforuntaley these things happen though it is far an few between... i think £10 and some vouchers is fine.