I'm planning to repoint the chimney on my Victorian property whilst i have scaffolding up. Thinking of an nhl 3.5 mortar but I'm aware that this is the wrong time of year for lime mortars. I'll be able to keep it covered with hessian for a week, during which time the temp will likely dip below 5 degrees or get very close. After that we're heading into Winter so almost certainly getting colder. In other words, marginal conditions at best to or nhl. So what should i do? Risk an nhl mortar or would a weak cement mortar be better on balance?

Brick chimney emerging from slate roof, ie: no gable
Percieved benefits of nhl versus cement based mortars: softer and more porous mortar means better durability of pointing and less likelihood of damage to brickwork because of better moisture management

Greenpaddy's advice seems wise.

I am a novice but we did my son's chimney earlier this year with Lime(Summer)
We read that if mortar should be softer than the brick, I seem to remember.

He used stainless helibars and resin on some parts to make it stronger.
He had 3 bricks come down with the TV arial.
I think we were not that happy with the inside that had all the smoke going through it.