As no answer here, I asked a retired builder locally who said not to add lime this time of year as takes too long to dry in potentially frosty weather.  Instead, use a plasticiser and work on 3.5 to 1 ratio sand and cement mix - higher than normal (about 5 to one as a rule) due to the exceptionally hard brick used in the construction of the chimney.

Known in the trade as bellringers - hit a wall made of them and it rings.  

What a nice guy!

John D

No, the rain here is lovely and warm at the moment..........

Had to take the chimney down to about a foot to 18 " below roof level in the end and rebuild -- just couldn't find anything solid above that - I  had to tap just one brick lightly with a hammer, otherwise I pulled it apart easily with one hand. Worrying.

Thankfully all the flashing was reusable, and have saved every single brick,  which cleaned up nicely.   The top course - different to the rest - are however badly weathered and will be changed.

I am currently a foot above roof level, so all is going well!

John D