Fix as follows :

1. Insert wheel arch liner and position correctly. Drill all holes (as marked)  and insert screws loosely

2. Remove liner and paint each hole (to prevent future rust)

3. Re-insert liner and partially insert screws & washers then Spray screws with aerosol underseal.

5. Tighten screws, spray along join between liner and outer wheel arch lip with underseal (I used Hammerite underseal with waxoyl) and wipe away excess.