Hi Spangle - welcome to the Forum.
What a ridiculous situation! I'm afraid you need to forget that you are British And Don't Make A Fuss and go back to the school again.
The school probably has a homework policy somewhere (try their website) and that probably includes a reference to marking.
Regardless of that, if your child's other books are being marked, then the simple question you have to ask is: "If all the other books are marked, marking of homework is clearly considered to be important, so why is marking not important for Science?".
Write a polite letter or email to the Head, stating that regrettably, you once again have to draw this problem to his/her attention. Ask what the school's policy is on marking of homework. Ask why Science is an exception. Make things a little uncomfortable for him or her, but always be very polite. Your child should never suffer provided you remain polite - I had to write to my son's school last year about a dismal failure in one area of teaching, but despite that I am still on excellent terms with the Head and the Head of Year.
I completely agree with your friend the Science teacher - this is just sheer laziness on the part of the teacher concerned. I have a feeling that Guest55 (who is a teacher) may be along with some suitably "pithy" comments on the matter before too long!