The crew that installed the bath in the house we are living in did a poor job (but in the face of considerable difficulties) and it leaked water underneath that couldn't drain away properly. We recently took in out and made some changes that should solve the problem but putting it back is proving tricky. The issue is that getting the bottom supported just right is very difficult - too high by even a millimeter and we'll get leaks around the edge when showering and too low will probably prove too much for it and one of us will fall through it before too long.

There is 17cm from the floor to the underside of the bath and I originally thought to use some blocks and put mortar in a bag (with some small holes) to contain it on top - the idea being to let it mould itself to the perfect shape.

One of the people building our new house suggested that ready mixed asphalt would be better than mortar. Any thoughts, suggestions welcome!

It's difficult to understand your construction difficulties but mortar is the local favorite for just about everything - so I would let them proceed with that. Asphalt in your bathroom??? - does not sound good to me.

The acrylic bath in my work accomodation was replaced about 6 months ago. The workmen used a bed of fine sand to support the underside. I've had no problems with the bath since then.

Sand seems like a good idea but I'd have to put that in bags or it will end up in the drain. I'm nervous about the smell and oily aspects of asphalt too.

why would the sand end up in the drain?

the bath outlet is in the drain surely?