Halfords just replaced my chainset and it is a 10 speed one, but my cassette is 8 speed. Am I ok?
In my experience this is a non-issue. The only difference between 8,9,10 speed chains is the exterior width not the interior. That matters a lot on the rear cassette, but not much if at all on the front. From this article the difference in the exterior width of 8spd chain vs 10 spd is 0.9mm. On the front that just doesn't make any signficant difference. 

If you had a triple crankset and really short chainstays, you might run into some issues, but with a double and your bike, it should just work. 

I'd say take the bike for a test ride and if you're happy with the way the front derailler shifts and it is capable of cross-chaining then I wouldn't worry about it. At this point insisting on an 8spd replacement will only get you a very low quality crankset.