Some thoughts:

It sounds like there isn't enough fuel pressure. You might want to start by considering the fuel filters ..... I believe you have three of them at least on W123's there are three .... one in the tank, one plastic inline, and the main canister. If your fuel filters are new/ok then maybe your fuel pump itself is worn and not producing enough pressure. When you pump it by hand and it starts fine that seems to suggest that the mechanical pump mounted on the side of the injection pump is weak or leaking pressure when it sits. Another possiblity is that if when it started last that it idled high or uncontrollably ...... perhaps you inadvertantly filled up with gasoline? It happens. If it is gasoline in your fuel, you'd have to drain the tank and replace all the fuel filters and use the hand pump to clear the injection pump of gasoline. After doing so it should start back up. Good Luck.

David Roseman
Long Beach, Ca