Hello Peter,
英语&英文 sometimes are interchangeable. But 英语 is English language, always focuses on speaking English language and emphasizes on a kind of language. 英文 is a specific style of English, always focuses on English characters.
Here are some examples about 英语&英文:
1. --- 你的母语是什么?
--- 我的母语是英语。
---What is your native language ?
---My native language is English.
2. --- 这篇文章是用什么语言写的?
--- 这篇文章是用英文写的。
--- What language was the article written in?
--- The article was written in English.
I hope my answers can be helpful for you, if you have any problems about Chinese, just let me know.
^_^ Anna