Other then the light thats lit is the dishwasher still working?
Ok if the dishwasher is working fine and its just the light for the rinse aid thats on then its either a loose wiring connection on the reed sensor on the dispenser or the sensor itself at fault.
What you need to do is remove the outer door panel.
From there you will see the back of the dispenser. On there you will see 4 wires. 2 will go to the coil and the other 2 which is the thin wires are for the reed sensor which detects the rinse aid in there. Check the plug is pushed on tight.
If it is then the dispenser unit itself is faulty as the reed sensor is part of the dispenser. In this case you would need to replace the dispenser unit to clear this.
But its weather its worth spending the £50 roughly on it when it still works ok. As a faulty reed sensor wont stop it from working it will just keep the light on when it shouldnt be thats all.
But check the wiring plug and see what you find as it may just be a loose wiring plug.