I have been using Japanese for Busy People Vol. 1 but I have been given a book grant of £35 for the bookshop. Should I buy the JFBP workbook and the Berlitz workbook? Or is there another better book? There are 3 shelves FULL of Japanese study books, can anyone recommend?
Japanese For Busy People workbook is OK but its more of the same exercises/tests as in JFBP 1.

I really like 'Genki'. I have no idea if it's in print or not, and I never reached the end, but what I did get through was really good.

The best language program I have ever used is the Pimsleur Language Program. I am currently on the Japanese program as a matter of fact, and it is fantastic. I can already carry conversations with my friends from Japan. Since it's completely audio, you learn pronunciation extremely well and FAST. The program consists of 30 lessons of 30 minutes each. They're so convenient, you can listen to them in your car, on walks, the bus, or just relaxing at home. The only downside is you don't learn to read Japanese. It is quite expensive though. I recommend going on ebay or amazon to find cheaper used versions, they work JUST as good as new, I promise.

You did mention Berlitz, which is a great program as well, but I have found Pimsleur gets you speaking and remembering much more naturally. There's no tedious memorization, it all comes natural like when you were a baby learning English.

I hope this helps you!