have you got a photo ?
Sorry Mike no photo as i have laminated some pine over the top to hide, and it's back in the shed. I have heard you can use a damp cloth and an iron but only seen this used to take out small indentations.
I suppose the temperature difference was just too much for the wood to handle. I will be using shellac to finish but now not sure whether to do this in the cold, or warmth?
It's not heat or cold, but lack of humidity, which causes cracks in wood.
Just shooting around in the dark here without a pic:
Boards can "check" on the ends. The solution is to cut off the end of the board and therefore any potential checks before using a piece of wood on your guitar. I think generally the check is self limiting and won't move any further, but it can be unsightly (or cool depending on your point of view).
In general, you would want to rehumidify the crack to get it to close up first before laminating a brace over it. Then make sure you keep the guitar humidified during the winter.
You can use a Dampit or other guitar humidifier in the case; I simply take all my guitars prone to cracking down to the basement for the winter.
I would keep it warm for the shellac, so it will cure in a timely manner.
Extremes between heat and cold can cause cracking in a lacquer finish, but shellac does not crack. I had a 100 year old Gibson (made back when they were still using shellac) with no finish cracks!
Thanks for the advice Skeesix, and the heads up on keeping the shellac warm. I shall keep the guitars somewhere cool from now on.