Correct source code for medical expenses on the income tax return?

You can rectify it by clicking "request correction" on your income tax working page. The source code is 4020.

Will this affect the outcome of your tax return, as my total due is now zero and on the previous assessment I got well over R2000 and they requested supporting docs, while the total not covered by the fund is almost R22000

It would depend on the level of you income. Please read our blog on tax deductions for Out of Pocket expenses to get a better understanding of how it works.