I've had good success by raising the rear axle height, and loosening the diff slightly. 103's are upset by weight balance easily, By raising the rear height, your shifting your weight balance forward and increasing front grip. Only make 1 change at a time so you can see how each change affects the car.
When setting your diff, Think of it like a clutch pack LSD in your car. By locking the rear wheels you'll induce understeer on turn in, but increase drive on exit, By running a loose diff, you'll have great turn in, but tend to light up the inside rear wheel on corner exit.
Turn in understeer is usually the last thing I'll address when setting up unless it's diabolical, then I'll loosen the diff right out until I get the chassis well balanced mid corner, then tune the diff accordingly.
If your still a bit unsure, Read Carroll Smiths Cause and Effect, It gives you a really good starting point and a good understanding of car balance and how certain changes will affect it
Cheers
Aaron