"iie. mimasen deshita" not "iie. eiga wo mimasen deshita" ???
I am doing some exercises in my workbook and the question is:

Mari-san wa ketsuyoobi ni eiga wo mimashitaka

The answer is: iie. mimasendeshita (she actually wrote a letter)

Would it be wrong to use 'iie. eiga wo mimasendeshita' as this sounds more natural to me?

I have heard a lot of 'inferred' information can be dropped from sentences in Japanese, is this an example of this?

Any info gratefully received :), and apologies for the romaji formatting, I don't read much romaji to know how to split the words well.

Question

"iie. mimasen deshita" not "iie. eiga wo mimasen deshita" ???
I am doing some exercises in my workbook and the question is:

Mari-san wa ketsuyoobi ni eiga wo mimashitaka

The answer is: iie. mimasendeshita  (she actually wrote a letter)

Would it be wrong to use 'iie. eiga wo mimasendeshita' as this sounds more natural to me?

I have heard a lot of 'inferred' information can be dropped from sentences in Japanese, is this an example of this?

Any info gratefully received :), and apologies for the romaji formatting, I don't read much romaji to know how to split the words well.

Answer

Yup! The answer should be: いいえ、みませんでした。
It's just similar to English, when you ask someone: Did you watch the movie last night? 
Normally, the answer will be: Yes, I did/No, I didn't. (Not: Yes, I watched the movie/No, I didn't watch the movie; except in case you wanna emphasize or give additional information).

Verpassen Sie nicht die Gelegenheit, bequem von zu Hause aus eine Sprache zu lernen. Stöbern Sie in unserer Auswahl an erfahrenen Sprachlehrern und melden Sie sich jetzt zu Ihrer ersten Unterrichtsstunde an!