Combine text with max date for dynamic title

I have done this before, but i've clearly forgotten how to build a simple measure because my syntax is not working. 

I want to create a measure which says "Online Monthly Service for month year" where 'month year' is the max date in my Date dimension table. Simples. 

Here is my syntax, can you please tell me how to fix it? 

Report Title = "Online Monthly Service for " && FORMAT ( MAX ( 'DimDate'[Date] ), "MMM YYYY" )