If anyone else has any experience of this please answer here so others can benefit too

You need to show that you have studied a similar skill or art in your own country previously - I have studied martial arts for 20 years.

I first found a martial arts teacher who was kind enough to offer to teach me.
I got a letter from him saying so and he also completed a form that is part of the Cultural Visa application.
I then had another Japanese friend sign as a guarantor (it is a financial guarantee - should I leave the country with unpaid bills etc.)
I then got original bank statements sent to me from my home country to prove that I could support myself for the year. (maximum Cultural activities visa is one year)
If they suspect you intend to work they probably wont give you a cultural visa - so I would not mention work (teaching private students is not legal on most visas anyway).
Careful mentioning the girlfriend too.. from what I have seen it rarely helps and might complicate things.
It took about 2 months for them to process it.

Hope this helps