The court does not apportion blame when it comes to dividing the marital finances. You have been married for 4 years and the period you were together before you got married would be taken into account and hence, 12 years is a long marriage.
It appears that there is very little equity in the property other than that which was provided jointly by your parents but you do say that it was a gift and not a loan . If it was a gift to the two of you, then the £18,000 or so of equity would simply be split down the middle.
There are no children and no other assets and you both earn the same so there is no maintenance to consider.
As there are no dependent children, then the only argument is over whether he does get half of the equity, including your parents gift or whether he gets less or nothing. The argument would appear to be over the sum of about £9000 and it would be easy to gobble up far more than that in legal costs over the argument.
It would be completely different of course if the gift was not a gift but a loan and there was proof that it was a loan because then, you would be able to deduct your parents money before paying him his thousand pounds or so. Not really worth arguing over.
If the gift was made to you in isolation and the property was your first house purchase within the marriage it’s unlikely he would get 50% but he would certainly get more than the nothing he brought in to the property.
.Can I clarify anything else for you? I’m happy to answer any specific points arising from this.