Slight update: All is still working well with EPG collector. I now have two collections running as scheduled tasks: One which grabs 99% of my channels using the Sky TV preset and another which grabs the ITV channels using the Freesat preset.
One thing I did notice yesterday, however, was that the search function in the EPG was no longer working and hence series links had fallen over. Even though WMC was set to index automatically under Indexing Options, it looks like this was not happening. A quick Google suggests that this is a fairly well known bug.
I added a new scheduled task to force a re index by running...
%SystemRoot%\ehome\ehPrivJob.exe /DoReindexSearchRoot
...one hour after my EPG grab, and now it all seems to be working again.
Note: You have to add this by running Task Scheduler as Administrator (otherwise it won't let you save the new task), and the task itself has to be set to run as a user account with Administrator rights. I also have it set to run "whether user is logged on or not".