Feeling out of pocket on my expenses

Hi guys

I need your advice. I have recently started going away for my job. I am currently temping via Hays. I have already submitted 1 expense claim and I have another one to be submitted today.

When I finally got my payment it was only the net amount and not gross which I was expecting.

How do I go about reclaiming back the difference/tax back (At present it will be about £100). Is it via self assessment?

Thanks in advance.

Spencer