Can you  post a screenshot of your batch task condition?

Looks perfect, so I don't understand why it doesn't work. I use batch conditions for search and replace.

But if you want to have this operation simpler, use the same filter on a TMX export and then create a new TM from that export.

That sounds like a good plan. What programme would I be using to edit the TMX export? Any XML editor?

You export the TMX directly from Studio. To do this, open the TM in the TM View in Studio, create the necessary filter and save the filter. Now close the TM, right click its name in the list on the left and chose export. Set the filter for exporting segments. After the export has finished, you should see how many segments have been exported. You can compare this number with the total in the current Tm, it should be significantly higher.

Now create a new TM, using the old one for "Create from", this way you will have the settings in your new TM exactly the same as in the old one. Import the TMX there and you should be done. There should be no need to edit the TMX. Should you however like to edit TMX files, you can do that directly in Studio too, there is a special file type for that on the app store.

You export the TMX directly from Studio. To do this, open the TM in the TM View in Studio, create the necessary filter and save the filter. Now close the TM, right click its name in the list on the left and chose export. Set the filter for exporting segments. After the export has finished, you should see how many segments have been exported. You can compare this number with the total in the current Tm, it should be significantly higher.

Now create a new TM, using the old one for "Create from", this way you will have the settings in your new TM exactly the same as in the old one. Import the TMX there and you should be done. There should be no need to edit the TMX. Should you however like to edit TMX files, you can do that directly in Studio too, there is a special file type for that on the app store.