Thanks Ricardo, this definitely sets me in the right direction.
I just noticed that, when I hardcode the task URL, it works, but when I set it with an expression, it doesn’t.
For the error image above (POSTing to a url created with a expression), seems like the “task” value is getting lost and the POST fails.
The expression seems to be good (I copy and paste the results of the expression and hardcode it as URL, it works.)
Again, thanks for all the help and apologies but the solution doesn’t look obvious to me. 
Regards,