Dishwasher was checked by plumber and was told it had internal flooding . It started working as soon as he took of the side panel to check it out. I assumed that was the end of that, only now it seems to have the same symptoms, so I assume the problem has returned. What I would like to know is whether its worth fixing or should I cut my losses and buy a new one.

Its entirely fixable, and he should be called back for FREE to do what should have been done in the first place. Review this link, How to Fix a Flooded Dishwasher | Home Guides | SF Gate, and verify he checked all these things, and not just pulled it out and removed the cover. If he did, he might have missed something.

Could be a float switch, a drain solenoid, a crimped drain hose, clogged air gap, clogged pump and possibly others. We can't see the unit so only best guess. However, if unit is more than 8-10 years, just get another one, they are easy to install and relatively inexpensive.

Why didn't you tell us what kind of dishwasher you have? I could have sent you a repair manual and everyone could have advised you better. However, if you are inclined to buy a new one I will list the highest rated ones and prices here:
1. Kenmore Elite12793, 12803, or 12833- $1,350 (Different numbers for different colors) Cycle time= 145 minutes
2. Kenmore Elite 12783, 12763, 12773, or 12813- $1,200 Cycle time= 135 minutes
3. Bosch Ascenta SHX3AR7[5]UC- $730 Cycle time= 95 minutes
Other units cost more and are rated lower. I am sure you can find better prices by checking stores like Home Depot and Lowe's. The Bosch Ascenta is equal to higher rated units, but slightly noisier. This can be fixed by buying a sheet of Styrofoam and a tube of GE Clear Silicone caulk and then cutting pieces to insulate around the dishwasher, either on the surrounding cabinet or on the machine itself. Get a sheet as thick as will work, or get a 1" thick sheet and double it where you can. Then you can make your dishwasher even quieter than any dishwasher. Good luck.

Reference: Consumer Reports, July 14, 2014, Dishwashers fit for a crowd, Issue #44, Consumer Reports Magazine, Yonkers, NY, Page 45