I am trying to set a value for an NTP server in the DHCP advanced options on a 2860. NTP servers are option 42.

I have tried both an ip address and a url, in both cases entered as ASCII (e.g. "uk.pool.ntp.org" or "176.58.109.199")

However the client picked up what appeared to be random hex code.

Should I be entering the values as hex, and if so how do I translate?

looking at the RFC, I think the IP addresses would need to be converted to hex, for instance FF becomes 255 - you can convert it usually using a site that has decimal to hex conversion or you can do it using the windows calculator in scientific mode:
http://www.miniwebtool.com/ip-address-to-hex-converter/?ip=176.58.109.199

I think it should look like this when entered on the router