In response to the OP I can confirm there are not currently any TPN's directly related to I.T. Systems Engineering is in a more general sense and not specifically focused on I.T. The IET is currently reviewing its community engagement programme and there is a consultation process currently under way and I would encourage you to submit your response on this matter as this will aid the IET in its decissions going forward - http://kn.theiet.org/community-engagement/
You are right in noticing that the closest match is the Multimedia Communications Network is the one that deals with I.T related topics. Whilst their focus is largely on broadcast and communications, they have covered I.T and internet related topics with for example; a lecture on the 40th anniversary of Donald Davies paper on packet switching and a technical visit and tour around the National museum of Computing.
The IET is looking to expand its activity in the I.T space and the Information and Communications Sector is one of the ways. If there is a subject you would particularly be interested in having an activity on then please feel free to let me know and I can work with the Executive team and see what is possible.
Head of Sector - Information and Communications
Community Development Manager - MMC Network and V&I Network