Pete,
I updated mine last year when I bought the car, I think I paid around £80 for it on a website (SatNavShop). It arrived not on an SD Card but on a USB Memory stick and the supplier also provided a CODE to enter when updating (This is tied to the Vin Number of the Car).
The update took about 2hrs 30mins but works flawlessly. Sounds like you have been unlucky.
I think 2018 is still the most up to date you can get.