Alright guys, but it's by no means a clear-cut tutorial, and well technically - I didn't hack 1.2.42, but 1.2.33 then reverted back.
Firstly I had 1.2.42 installed and up to date. I also had Lucky Patcher's 'Disable signature verification in the package manager' applied in xposed settings.
What I did was install the 1.2.33 apk version over the top of 1.2.42, then I went into the data/data game folder and deleted (my memory is a bit fuzzy here guys) player.player.dat, play_stats.csv and possibly the player.HERO file there.
Then I (funnily) opened the update_complete file with a text editor and changed the version to 1.2.33. Opened the game, made a new character then...
I followed this YouTube tut exactly:
https://m.youtube.com/watch?v=aHLEKmeAT14 to get the gems and gold.
Finally I quit, installed the 1.2.42 apk over the top of the other, edited the update_complete file to read 1.2.4.2 and I was set.
I backed up all my files before doing any of this.