I worked out that not only was it my WiFi, but my network connection also - the only way to get them to work after waking from sleep was to reboot. Even worse, if you pulled the network cable out, then plugged it back in, it didn't work.
In the end (with incessant Googling and the fact this obviously isn't a unique problem, I came across http://fieldguide.gizmodo.com/the-worst-bugs-in-os-x-yosemite-and-how-to-fix-the m-1652690924
On that page, among other things, it tells you to clear your network setting files. I tried as per Allen's post above (to no effect), but this worked/fixed it all for me (from http://osxdaily.com/2014/10/25/fix-wi-fi-problems-os-x-yosemite/):
deleting all of:
com.apple.airport.preferences.plist
com.apple.network.identification.plist
com.apple.wifi.message-tracer.plist
NetworkInterfaces.plist
preferences.plist
from:
/Library/Preferences/SystemConfiguration/
Full instructions at osxdaily as above - do at your own risk, etc., etc.
Good luck!