pump up the tyres to a high pressure, and then cover with a solution of water and washing up liquid
bubbles will appear where air is leaking out. then shake the wheel trying to get the sealant inside the wheel to the area where the leaks are appearing.
thanks, i'll give that a go tonight. Only had to do that before with mtb tyres which aren't tubeless but I think the sidewalls are quite thin on the horizons and its getting through there.
can't see anything coming through valve hole and the rear is fine and it was done in exactly the same way (sidewalls)