I completed the first mission with a score of 76/100, but there was no "Next Mission" button to move on to the next one, am I doing something wrong, I cannot get past Mission 1 in the Diner? Many thanks.

I'd advise you take a look to your campaign difficulty level. Depending the difficulty you have selected, probably 76 is not enough to advance due to the harder requisites. Who knows, maybe that is the reason.

Greetings

 
To successfully complete a mission and advance to the next one, you must achieve a minimal score that is based on the selected game difficulty.

At the end of a mission, the required score appears above the score you received for that mission (see attachment).  If you reach the required score, the next mission will be available.  Otherwise, you'll have to retry the current mission to get a higher score.


Here's how your final score is calculated :

Mission complete :                 + 40
Suspect arrested :                 + 25
Suspects incapacitated :            varies
Suspects neutralized :              0
No suspect neutralized :            + 5
No civilian injured :                 + 5
No officer down :                  + 10
Player uninjured :                  + 5
Report status to TOC :              + 5
All weapons secured :               + 5
Unauthorized use of deadly force :    - 10
Unauthorized use of force :          - 5
Kill a hostage :                     - 15
Injure a fellow officer :               - 5
Incapacitate a fellow officer :         - 15
Fail to report a downed officer :       - 5

Thanks for your replies, I found a manual on GOG for this game and realised I started in the wrong place,my bad.