do you think £150 ($255) is expensive for a theory course? that takes you up to ABRSM grade 5 theory level so you can get on to grades 6-8

Try www.aaronpublications.co.uk

The course prices are cheaper and the teaching method has very good reviews

so what exactly does this course include? is it a self taught course with books, dvd's that sort of thing?

You get your own personal tutor (usually the author herself), work/modules sent via email,regular reports upon your progress and any of your theory questions answered within 24 hours. Plus it's a new approach to teaching music theory which has been highly recommended by Rhinegold (in their Music Teacher magazine), a journal for educational resources in the UK
See... http://aaronpublications.co.uk/theoryviaemail.aspx