Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
iOS: FaceTime is 'Unable to verify email because it is in use'
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
iOS 6 and OS X Mountain Lion: Link your phone number and Apple ID for use with FaceTime and iMessage
How to Set Up & Use iMessage on iPhone, iPad, & iPod touch with iOS
Troubleshooting iMessage Issues: Some Useful Tips You Should Try
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
FaceTime, Game Center, Messages: Troubleshooting sign in issues
Unable to use FaceTime and iMessage with my apple ID
How to Block Someone on FaceTime
My Facetime Doesn't Ring
Send an iMessage as a Text Message Instead with a Quick Tap & Hold
To send messages to non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
How to Send SMS from iPad
You can check the status of the FaceTime/iMessage servers at this link.
If your Apple iPad has telephone system capabilities (in other words if it can receive SMS text messages) it can send SMS text messages to any other device that can receive them. It doesn't use anything connected with Apple to do so, just your normal mobile phone connection.
If your Apple iPad only has WiFi, and no phone connection, then you should not have been able to receive an SMS message to start with.
Sorry, Simon, but that's wrong. The iPad does not have cell voice technology, so it is not natively capable of sending nor receiving SMS or MMS messages (SMS/MMS requires cell voice; they don't work over cell data). There are some third-party apps that offer this via a bridge technology the app's developer will offer, but I have no experience with any of those apps.