Using the text editor in iFile or an equivalent method, open:
/var/mobile/Applications/Safari/Library/Preferences/com.apple.mobilesafari.plist
Scroll to the bottom and before </dict></plist> add the text below, save and probably best to respring:
Code:
<key>UserAgent</key><string>Mozilla/5.0 (iPad; CPU OS 7_0 like Mac OS X) AppleWebKit/537.51.1 (KHTML, like Gecko) Version/7.0 Mobile/11A465 Safari/9537.53</string>
I set Mercury Browser to Mobile Safari (iPad) and went to whatsmyuseragent.com if you're wondering where the string is from.