I had another opportunity to test this last night. Upgraded the FG to 5.0.6 and the AP to 5.0.7. Set DTLS on both. No connection. Rebooted the AP, reset and reconfigured it. No connection. Set to " clear-text" again and the connection came up.
Went to bed grumpy.

Had another look this morning. Deleted and recreated the config on the FG. Still no joy.
Went into the " WiFi Controller->Managed Access Points->Managed FortiAPs" section to edit the FAP-14C settings. Changed the AP profile from " Automatic" to the profile I' d created. The AP then connected with a DTLS link!
There was only 1 profile that was suitable for the FortiAP-14C and that was being picked up when " clear-text" was set. Not so for DTLS. Not sure why?
It' s working now and the client has placed their order!
Thanks again for all the assistance.
NSE4