What is it in eno antacid that requires to have an expiry date ours has gone past expiry and i can't see what could date so quickly sodium bicarbonate sodium carbonate citric acid anhydrous?

Hydrated antacids may be less effective. The anhydrous sodium carbonate in Eno antacid powder would absorb ambient humidity as soon as it was opened.

However, barring inappropriate storage (excessive heat, broken packaging, wetness around the package) this stuff should last forever in individual serving packages. So, the expiration date is probably either to: encourage you to discard a still-useful product; or federal guidelines set some sort of standard maximum expiration period.