Peter Ashton is an agent with MI6.
Unlike most of the operatives of the British Intelligence community, Ashton did not come to them through the Oxbridge method, namely being from one of the two major universities. Instead, he attended a smaller institution where he became proficient in Russian and German.
This "small school" origin puts him at a major disadvantage with the old-boys-network because he isn't "really one of our kind".
After college he joined the Special Air Service, being wanted as an interpreter but the activity even in such a action-oriented group was limited and he grew bored enough to volunteer for service with the regular army in the Irish Troubles. A nine-month tour was survived but then there was no opening back in the SAS.
Leaving the service, Ashton spent a brief unsuccessful time as a civilian language teacher but he found the life even more stifling and boring. Then came the Falklands War and with it a chance for serious action. He returned to his previous life and his exploits behind enemy lines brought him to the notice of his superiors and at the end of the conflict an offer by the SIS for employment. Now he had a chance to use his experience, his language skills, and his desire for action to a good use.
As the series begins, Ashton is described as "a thirty-three year old middle-level desk officer of proven reliability." As the series progresses, Ashton's experience and his value to the Service grows steadily but one does not succeed in a literal cutthroat like his without making enemies both in the opposition and in one's own department and Ashton is no exception. There are more than one other agent who would love to see him fail. Luckily for Aston, failing is not a common occurrence.