mc23.lon.server.colt.net has no CNAME records.
Question and response
Question
dig mc23.lon.server.colt.net. NS
Answer
The response has no answer records.
Authority
server.colt.net. 1905 SOA ns20.colt.net. eudnsmaster.colt.net. 2013053135 10800 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0