MyGCSEScience is an excellent site I’ve just recently found. It contains 220 videos (at the time of writing) covering the entire AQA Science courses. The site is free and very easy to navigate. If you have students taking eparate sciences then they will be able to see videos giving complete coverage of separate Physics, Biology and Chemistry including the unit 4 option topics. Students of Core and Additional will also be able to view videos covering all topics in all sciences. This site is perfect for revision or setting homework from. Have a look and see what you think.
The image above is from here.