Windows 7 Logon/off events

Can anyone explain this in simple (and I mean very simple !) terms ?

Do you have the correct auditing enabled so that the 4800 and 4801 events are generated?

If you look here http//stackoverflow.com/questions/11385164/eventviewer-eventid-for-lock-and-unlock

The following MS article is linked https://support.microsoft.com/en-us/kb/977519

So, are you auditing for "Other Logon/Logoff Events"?

I need to identify when a user has logged on/off a PC

The event IDs you should look for logoff are
4634 An account was logged off.
4647 User initiated logoff.

and ID 4624 for successful logins

Nice list of event IDs across different Windows OS versions (page 20)
SANS Windows Logon Forensics

.. However there are plenty of 4624 ID's with Logon Type 7 - which does signify an unlock I believe….

Correct. Log on type 7 means "Unlock (i.e. unnattended workstation with password protected screen saver)"

..if the workstation is a member of a domain, at this point it’s possible to authenticate to this computer using a local account or a domain account – or a domain account from any domain that this domain trusts. When the user logs on with a domain account, since the user specifies a domain account, the local workstation can’t perform the authentication because the account and its password hash aren’t stored locally. So the workstation must request authentication from a domain controller via Kerberos. An authentication event (672/4768) is logged on which ever domain controller handles the authentication request from the workstation. Once the domain controller tells the workstation that the user is authenticated, the workstation proceeds with creating the logon session and a records a logon event (528/4624) in its security log.

What if we logon to the workstation with an account from a trusted domain? In that case one of the domain controllers in the trusted domain will handle the authentication and log 672/4768 there, with the workstation logging 528/4624 the same as above.

In all such “interactive logons”, during logoff, the workstation will record a “logoff initiated” event (551/4647) followed by the actual logoff event (538/4634). You can correlate logon and logoff events by Logon ID which is a hexadecimal code that identifies that particular logon session…

link and this is followed by

..After logging on to a workstation you can typically re-connect to shared folders on a file server. What gets logged in this case? Remember, whenever you access a Windows computer you must obtain a logon session – in this case a “network logon” session. You might assume that the logon session begins when you connect to the share and then ends when you disconnect from it – usually when logging off your local workstation. Unfortunately this is not the case Windows servers only keep network logon sessions alive for as long as you have a file open on the server. This accounts for repeated logon/logoff events..

which should explain your multiple logon/off events.