At the point when I had to tell Regis whether we would tackle that master vampire, or rescue Syranna, I chose the option
"Need to think about it first" I then discovered I could not meditate or fast travel at this point but the map directs me along the path to where Damiem informs me where Syranna has been taken.

First problem, I am supposed to be following Regis but because of my being undecided on which route to take he is not with me. (he did mention that when ready I could easily summon him) but not sure how. I am currently at the bridge where
the population are running in the opposite direction. Would having Regis with me make fighting the vampires ahead any easier?

I believe after speaking to Damien there are different paths to follow, each one resulting in different additional quests.
Has anyone experimented to determine the best way forward from this point.

Yes, Regis tells you your options when the quest starts (after you kill the bruxa): talk to Damien to try to find Syanna, or find a way to contact The Unseen Elder. While this quest takes place, you won't be able to fast travel or meditate. Either way, you'll be prompted to summon Regis when he is needed - don't worry about losing him! You should have pips on your map that represent either option if you told Regis that you weren't sure yet (you might not see both pips even if you tell him that you've made a decision - I don't know for certain)

As far as the best route, its hard to go into detail without spoilers. Suffice to say that what you do here has an impact on the end of Blood and Wine. I think you're best off doing what you think is right, and seeing how things turn out. If you don't like it, make a back up save and reload - its a long quest, but not so long that you can't replay it. If you really want to know, do a web search for 'blood and wine endings guide', and you should get all the info you need to know about your choices here.

Regardless of your choices, once this questline is over, you will be able to return to Touissant and continue questing and exploring.

I spoke to the birds who summoned Regis. Quest completed. .

Ah, sorry - wasn't very clear there. I mean that you should have pips on your map that represent the two locations for the branching paths of the quest: talking to Damien or finding the Unseen Elder.

To make matters a bit more confusing, you can talk to Orianna and do her quest before or after going to Damien.
You make the decisive choice when talking to Regis.