self employed work while also employed question

I am currently a salaried employee of a "Company 1".  I recently worked a day for "Company 2" just as a one time thing to see if I liked the company and they liked me.

My questions is do I have to pay tax the one time payment from the one day worked for "Company 2"?  Because they've just paid me directly as a locum day worked and not taken out any tax on me.

Thanks