Question

why is symbol S for sulphur but Na for sodium and Si for silicon

Answer 1

The Latin name for Sodium is Natrium. The symbol came from Latin name not English name. 
Same for Iron (Fe from Latin Ferrum) 
But the symbol for sulphur and silicon came from English name.

Answer 2

S is first letter of Sulphur
Si are the first two letter for silicon
Na are the first two letters of Naturism the latin name for sodium.

Hope it helps....