Programming :: Best Way To Transform XML Format?

I'm looking for some expert opinion on sed/script to work out the best way to transform one xml format into another however there are a few complexities around translation.

The extra complexities are to:
1) Take the start and stop time (YYYYMMDDHHMMSS) and convert to start time to unix time plus output the difference in seconds between both times.
2) Oid, tsid and sid are found by looking up an external file and finding the value against the channel. For example one of the lines in the file will be 2:806:27e2=channel1

Is there any way to write piped sed commands that can do this? If not, any ideas how the script should look like?

Input File
Code:
<programme start="20100910060000 +0100" stop="20100910061000 +0100" channel="channel1">
<title lang="en">This is the title</title>
<desc>This is the description</desc>
</programme>
Output File

Code:
<service oid="0002" tsid="0806" sid="27e2">
<event id="0">
<name lang="OFF" string="This is the title"/>
<text lang="OFF" string="This is the description"/>
<time start_time="1284098400" duration="600"/>
</event>
</service>

Look up file for oid, tsid and sid
Code:
2:806:27e2=channel1
2:756:37a3=channel2
5:4a06:42e5=channel3