Thought I would google the problem and it's pretty common in ios7. Apparently it will download a high quality Siri overnight on wifi or you can follow these instructions that I found...
To fix this problem, follow these instructions (make sure Wifi is on first though):
Tap Settings >
Tap General >
Tap Accessibility >
Tap VoiceOver >
Scroll to and Tap Languages & Dialects >
Tap - Default Dialect >
Tap your preferred language (British English) > Turn on Enhanced Quality which is at the bottom of the same page.