Thanks! for anyone interested Universal have come back to me and clarified. It was confusing as the website mentioned up to 4th April (although concerts were only listed to 31 Mar) and also access being blocked for the entire day to USF on concert nights (not just the evening).
Response from Universal below -
"The Seasonal Pass is blocked out at all three parks between April 12 - 27. In addition, that pass would be blocked out at Universal Studios Florida on concert nights (Feb 16, 17, 23, March 2, 9, 10, 16, 17, 23, 24, 30, 31)."
In terms of going the seasonal pass route to get the APH rates any other gotchas to be aware of ? Although we are going to use it as a one time only deal, the reduced hotel rates and other discounts look worthwhile.