I'm on the mission "Playing For Time" but I keep getting killed in the bit where motorbikes are chasing the car I am in. But the game keeps dropping me back to the heist mission.

When will I be free to roam around and do side missions again?

a lot of missions tend to put you in a state where you are stuck until you finish that actual "scene" and that mission completely.
the game autosaves throughout the missions so selecting continue or reload from the main menu starts right back in the mission.

if you're in the middle of a mission you just can't finish and are stuck during one of these "scenes", you will need to load a save from before the mission started to be able to enter other missions.

Ta thanks, I thought these missions were all up to you what to do but I kept being pushed from one to the next with no free time between them.

yea its pretty straightforward not so much of openworld
and choices dont matter at all :(