I've looked on the website and there's just a e.mail feedback form, no numbers or anything and their head office is in Texas.
http://www.drpepper.com/
If you look here there is more info as to who owns it and distributes it where...
http://en.wikipedia.org/wiki/Dr_Pepper
You could try Cadbury Schweppes to see if they can help...
Consumer Relations Department - UK
Cadbury Trebor Bassett
PO Box 12
Bournville Lane
Bournville
Birmingham
B30 2LU
UK
Tel: +44 (0) 121 451 4444
Fax: +44 (0) 121 451 4192
If they can't then they may be able to guide you in the right direction.
If they ask you to send it to them then take photos before you do, send it recorded delivery and give them a reasonable time limit in which to respond. Keep a copy of whatever you send with the track and trace number for the recorded delivery. You could threaten to go higher if they don't respond satisfactorily.
If you don't get any job with CS then try the shop you bought it from, you may get a better response if it is a large one like a supermarket.
You can also try environmental health at your local council, there is some good advice here...
http://www.brighton-hove.gov.uk/index.cfm?requ est=c1001212