Sandy floor great response saves the customer money and to be honest Ufh isn't that great considering all the time it takes to heat up. If customer does persist, then if the sub is chip board lay backer board and screed over, I hate screed master 2 but manufactures warrant it on wooden sub floors. Then return for a second coat or use f ball 700 and check no cables have lifted. Always allow for two screeds as most times you will require it. And don't forget high temp adhesive as if any issues occur including damp or water leaks that are not even your fault then the manufacturer won't give you the time of the day.
I did my mates toilet and shower in karndean tiles last year, used F46 in one and HT in other. None of the rooms has gapped. Not sure what that tells me.
