It is always the starting point where the marriage is longer than a few years and where there are children involved - but it is not necessarily the end point and the ocurt does take account of contribution and needs.
I am not certain what you mean by liability for half of the mortgage.
If there was a divorce then it is the net equity that would be counted as part of the assets - not the value of the flat itself.
However since yours in not a divorce situation there is no reason why the matrimonial home should not be transferred into joint names; and why your ex should not apply to be joined on the mortgage and the flat be placed in joint names as well.
Then you can each write Wills leaving your share of the properties to be held on trust for the children in the event of your deaths.
I am not sure which part of this you wish to have further information on - but if you confirm then I will try and find some links for you