Thanks for the replies, i have found another crack similar to the above between 3 and 4 now too.

The missfire was only up top in the rev range, so it may not be related... but the cooling issues would be due to this I would atleast think ? as the cooling system is under pressure.. im guessing under load the combustion pressures would drive the coolant away from entering the cylinder..

The car was starting to miss and carry on badly when i was starting it etc which i thought may have been coolant in the cylinder

On the gasket there was evidence of some damage where the first crack was and also around one of the bores on the gasket