Use the direct messaging system in UKB... just below your avatar (if you have one) is a little message bubble icon.... clicky and...
Talking to Messaging yourself is the first sign of senility Tom.
Click on either of these icons underneath the person you wish to message....
If they are online and logged into UKB...
If they are not logged into UKB it looks like...
Choose a 'Subject' for your message and write whatever it is you want to (you can also add other recipients if you wish to send a message to multiple people).
It can be a good idea to ensure the "Save a copy in my outbox" tick-box is selected as you might want to come back to the messages at a later date (a while ago someone was asking how to view old messages but they never used this option, which can also be set globally from somewhere in your account profile).