When writing a tv review can I assume that the reader has watched the programme?
No, you cannot assume the reader has watched the program. For example, I watch very little television and would likely not have seen the program you are reviewing. Therefore, you must provide a quick summary or work into your review enough of the program so that a reader can follow what you are saying. Maybe the reader normally watches the program, but missed either all of this edition or maybe part of it. Either way, your review must be comprehensible by anyone who reads it which means that they must understand the program to which you refer especially if they have questions or don't agree with your review.
I feel that when writing a tv review or any review, one must provide background knowledge. It is inappropriate to assume that the audience has watched the show or read the book. Bring the reader up to date. Provide some history. They may not be familiar with the episode or with the author's work. I feel that if you review a show or book, without providing some background knowledge; the review would be regarded as incomplete.
You might also consider finding the right words to include both categories of people who watched it and those who did not. What if the reader who watched it finds your review boring? Find the right words to avoid both situations. Make the review interesting for both kinds of reader.