The Piglet Files, on ITV (UK)
IMDb; Wikipedia

This is something I used to watch on public television. I don't remember it well, but I know I always found it quite amusing, and I'd like to see it again sometime. I probably never saw the whole series, though. Anyway, there was this university teacher named Peter Chapman (played by Nicholas Lyndhurst, who I'd later see in just a bit of "Goodnight Sweetheart," which I'd also like to see again sometime). At the start of the series, MI5 forces Chapman to work for them in their technology division, though he also ends up going on missions with his fellow spies (who are somewhat bumbling). He was given the codename "Piglet," which he didn't like, of course. Anyway, he has to lie to everyone outside MI5, including his wife, Sarah, to prevent them from learning he's become a spy. The only other character I really remember at all is Major Drummond, who was played by Clive Francis (whose work I've enjoyed in other things, such as May to December). I also seem to vaguely recall MI5 having a rivalry with MI6. Anyway, wish I could say more.