I finally got this working. It is SPI, not I2C, despite the labeling of SDA and SCL. For the benefit of others, here is what I did.
SDA needs to go to MOSI, SCL to CLK. Then DC, RES AND BK to whichever pins you want.
I was trying to get it working using this library - https://github.com/juj/fbcp-ili9341
No matter what I did, it didn't work. This is not the fault of the library.
The problem is the breakout board for this display doesn't have a CS pin. Some displays will work without according to the library, but not this one. I've read some people have taken the display off and put it on a new breakout board - this seems a waste of time and money considering the price. Better to get one that has a CS pin in the first place.
After many hours of testing (and the above library requires SPI to be turned off as it uses its own, but others require it to be on, and while testing I sometimes forget to switch it back on...) I got it working before collapsing in to bed at 2am, happy but tired!
I ended up using this library- https://github.com/solinnovay/Python_ST7789
I don't need the fast display speed of the first library so it will be fine for my needs.
But I wouldn't recommend the board. No documentation, incorrect pin labeling, no cs pin, description says 'oled display, no need backlight' but so far if I have the backlight off the display goes blank? Overall, a very cheap display that wasted about 8 hours of my time. I'll use it for my project now it works, but if I could go back in time and spend 10 more on a better one, I would do it in a heartbeat!