I have to travel into London every day from Hitchin. Advice please?

Hiya. I have to commute to London for my new job - sometimes to Liverpool Street and occasionally to Leicester Square. My job starts at 10am, so I guess that I'll be travelling at peak time. I won't actually be travelling around that much once I've got to my job (ie not around London loads). The cheapest way I have found to do it so far is to get a 1-6 zone travelcard from Hitchin, which is 18 quid a day, or 96 quid for 7 days. Is there any cheaper way I could do it please? The rail staff aren't that helpful, and my understanding is that an Oyster card won't be any help since I'm not starting from a London station. Also I'm 26, so I can't get a Young Persons Railcard. Could people with experience of commuting in and out of London please advise. Many thanks.