Back from my days of doing Edexcel GCSE statistics, I used Pearson's correlation coefficient on top of the techniques you used. It's not terribly complicated, but it takes some work to use - best to confirm your calculations using an Excel spreadsheet.
Also, there's more information in the following article.
http://en.wikipedia.org/wiki/Correlation_and_dependenceThat said you may want to look up books which cover a broad range of statistical techniques. A friend of mine did this to improve his coursework and got marginally better scores than me in GCSE statistics, but that's entirely down to you.
It should be noted that these things may not be in your syllabus, but if you are doing coursework much like I was then top marks are achieved by doing things which are related to, but not explicitly covered in, the syllabus.
I hope that helps.
Darren