I emailed you recently to let you know of a party my company was holding and that there was a client of mine going. 

When I read that sentence, I thought you had invited me and you were just e-mailing me to tell me off, lol.

Anyway that event was last night and i received an email from him saying that he couldn't make it. 

The fact that he e-mailed you to tell you, meant that it mattered to him, that he gave you an explanation.  What he could have done, was not turn up and given you an excuse, if you raised the issue with him.

he sent me an explanation of how he was going away on Friday but his plans had changed and he had to leave on Thursday night, as his friend was driving to their weekend location. 

This weekend away could have been very important to his life and could have been pre-arranged for a long time.  It might have been a once in a lifetime event that if missed wouldn't happen again.  Whereas meeting up with you can wait, he can do that at any point, whether at a party or on a date.  Those are the options in a blokes mind.

He said he had planned to go and have a good drink up. he said he was really gutted and wanted a catch up with me. 

He wouldn't say that, if he didn't mean it.

I replied back with "Ah shame, tell your friend he's depriving you of a kiss under the miseltoe"!

Good answer, let him know what he is missing.

To which he replied "With an offer like that i'll cancel my trip". 

That reply would not have come to his mind, if he didn't want to be there with you.

i have another party in 2 weeks which he is coming to.

So, fair enough, this will be the deciding factor about whether he fancies you.  Will he make his excuses and miss the night again, which would be too much of a coincidence, or will he turn up and you both have a great night together.  He now knows where he stands with you, he should at least expect a kiss under the mistletoe.  So if the thought of that frightens him, he will miss this party, if the thought of that turns him on, he definitely won't miss it.

Now does he sound interested from this exchange of emails? Or is he just making an excuse?

At this point I would say he is still interested.  Why would he bother contacting you, if he wasn't interested? Why would he miss an office party just because one person there fancies him?  Why would his responses to you all be positive?

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