Tubeless tyres deflating - serious problem or just because I don't have sealant?
Is this just because I haven't got around to adding sealant yet...?



Yup.

The sealant plugs the very small air leaks in the casing and between the rim and bead. The tire will leak down substantially faster without it.




Despite the fact that it's not the subject at hand, it's worth reinforcing what brendan said: make sure you're running tubeless specific road tires for road tubeless. Standard beads- both steel and kevlar- stretch, and without a tube to bind them in place they will stretch enough to climb over the rim and blow off suddenly and catastrophically. You can get away with non-tubeless specific tires at the lower pressures used by mountain bikes- Stan's made their initial mark on the industry by providing conversion kits for non-tubeless rims and tires. But you cannot get away with that at road bike tire pressures. Road tubeless tires use a carbon bead that doesn't stretch, and therefore doesn't blow off the rim. That is the critical innovation that took so long to come around and make road tubeless possible.