IP codes always have at two numbers (they can have letter suffixes too).
e.g. IP44, IP66.
They may have an X in either position
e.g. IPX4, IP4X.
The first number means access protection against solid objects (hand, finger, screwdriver, wire)
The second number means protection against water (dripping vertical, dripping slanted, spraying, splashing, jetting, immersion).
"X" means "no protection of this type".
So your IP24 heater is protected against a 12.5 mm finger and splashing.
Have another look at the requirement you had. Do you know which bathroom zone it will be fitted in? Look at the pictures on the 8th page of
http://www.niceic.org.uk/downloads/NL139supp.pdf
If not: Is the heater above a bath or shower? If not, how far away?
How high off the floor is it?