"A valued member of a diverse team, you’ll follow subjects of national security investigations by foot and by car. Your observation skills, quick thinking and ability to fit into your environment means you’ll be able to make a big contribution."
MI6 do wonderful full page adverts in most of the major UK newspapers (Economist, Times, Independent etc.) that point out that they'll need to create a convincing cover story for you, but that you'll be able to tell a few trusted friends and family.
Made me wonder though, how MI5 do advertise. Found this: https://www.mi5.gov.uk/careers/mobile-surveillance.aspx
"A valued member of a diverse team, you’ll follow subjects of national security investigations by foot and by car. Your observation skills, quick thinking and ability to fit into your environment means you’ll be able to make a big contribution."