Hi Garth,
I tried this with some success, maybe im missing something obvious, you able to advise any further or one of your famous blog posts
so I've edited the parameter and added this expression =datepart(DateInterval.Month,dateadd(DateInterval.Month,-1,Today))
If I then run the report it will pick up the previous month which is good, tested this in April and today being 1st May, showed month value correctly. Tested hiding the month parameter and not hiding it.
Now the issue is with the subscription, now states "The subscription contains parameters that are not valid" - if I show the parameter then you have to select what is available as values, so there would be nothing for June until we are in June.