Last year I installed the free DLC which was separated into 16 files, New Game + being one of them.
I installed all except this one as I got a message that my system was not compatible, or something like that.
Gog has now incorporated all the files into one download which I have just done and installed all successfully.
This would include all that had previously been installed.
On launching the game there appeared a message that I needed to restart the game then select New game +.
As I am waiting for the next expansion pack I do not wish to begin a new game.
Can I launch New game + from my current position in the game or do I have to start right from the beginning again?

To play NG+ you must either start from scratch or import a saved game from *after* you finished the main story. This usually means saving your game when you are transported back to Kaer Morhen after beating the game. You must load that save, because others won't be compatible.

I think I will give this new game + a miss after viewing a video where someone on level 36 kept on getting killed by the White Orchard griffon near the start of the game.

That would be a shame, to never try it just because somebody else finds it difficult. I've played NG+ without problem.

Based on previous videos I have watched, like fighting the Caretaker, others seem to cope much better than me.
I died many times before killing the Caretaker. Possibly old age has taken its toll.

Mhm, but it always depends on the difficulty as well, doesn't it? I got the impression that many people feel the need to go for NG+ on Death March, which is fine, of course, but if it turns out to be too hard, why not go for an easier mode? No need to torture oneself, it is a game after all ;)
Personally, I think the game (especially the beginning) plays easier in NG+, simply because you already have a fine arsenal of weapons, potions, food etc.
Just go for it :) (Especially fun since you get some fine usage out of your HoS-reward.)

You made a good point. I have already started Fallout 4 so may replay the Witcher at a later stage.