wg mnie-
Dlaczego Mrs Kate Ashby, - albo Dear Kate, jak sie kogos dobrze zna, albo Dear Mrs Ashby, - i na koniec - Yours sincerely.
...I am writing to you...(po co ten poczatek zdania - przeciez widac, ze piszesz) czy nie lepiej zaczac - I would like to apply for the job in ...
I have been working as a waiter....- a moze - During last summer I worked as a waiter.....so I am familiar with various kinds of foods.
...so in my opinion...(ja to widze ze po prostu masz 'high opinion of yourself') - moze lepiej - I feel that I am a kind, friendly.....
*I am not going to study for next year....lepiej - I have decided not to study next year (nie trzeba 'for') albo - ...decided not to continue my studies next year
...a good qualification albo good qualifications...