You could try using mysupermarket .co .uk - it compares prices of a particular item for the big four and Ocado, also Aldi but that doesn't do many brands.   Try searching for  "chocolate multipacks" or "selection box". The offers change from Tuesday to Wednesday, sometimes they go on for a month or more but they always end on a Tuesday.

Also, have you considered Wilkinsons?  They have very good prices on Christmas goodies, often better than the supermarkets.

This is a list of stores when I typed in Exeter in their storefinder:

http://www.wilko.com/page/store?searchbox=exeter&submit=search

And - just a thought - seeing as it's Christmas, you could try appealing to Tesco's (or other) better nature (lol!), and just asking for some.  Just ask for the store manager. They're always banging on about how they do things for their local community - here's a chance for them to put their money where their mouth is.