Hi Nick
I presume from what you've said that your water has mixed with your oil and currently have 'mayo' on and around your oil filler cap. Personally if that was me I would be trying to find a replacement head or a complete engine as mentioned above by other members, or you could maybe find a local company that can pressure test your head and do further investigations on your block if indeed you have cracked the bottom block. If you were intended to sell the car, I would have to agree with Paddy26's advice its probably quicker and cheaper to get it repaired.