I gave it a go and unfortunately it doesn't sync to ST low or medium res, only high res. So if anyone else is wondering the EIZO FlexScan L350 doesn't work with the ST.
Thanks anyways, was cheap enough to be worth a try.
I also got one of those little adapter boards that plugs into the ST monitor output to provide a 15 pin S/VGA output. That works OK but I didn't realise with my Mega ST the floppy drive port is too close to the monitor port to allow the adapter board to fit if you have an external floppy also connected. So not doing too well with my 2 latest ST hardware purchases, haha.