is there a resin product available that in a single pour can attain a finished depth of 40mm or more and leave a perfect 'glass clear' finish or would it require multiple pours and polishing once cured ....

Depends what you are pouring it into.  Our GlassCast 50 will pour to 50mm thick in moulds.  However in wood, it is limited to 25mm per pour to prevent resin overheating. 

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant