The other day, I watched an old MI-5 episode...

Cafe Society
1

…in which Tome (the glom) got cashiered.

Is this for good, or does he come back?

2

I assume you mean Tom Quinn, and cashiered means fired. If so, then yes. His final episode is this one:

According to IMDB, he hasn’t appeared again after that:
http://www.imdb.com/character/ch0026402/

As we the last season broadcast was season 6, and his last episode was in season 3, I think we can assume he won’t be returning.

3

I think we can also assume I’m not a regular viewer of MI-5.

Thanks for the answer, silverfish.

4

Spooks is a great show, but don’t get too attached to anybody.

5

Of course, the worst aspect is when he asked the payroll secretary for his final check and she disavowed any knowledge of his activities.