Thanks for the link. I'm not quite sure what you mean by need a Drott not a JCB, since I don't know what a Drott is.
Anyway it would seem I have the right base, 100mm slab on top of at least 100mm of old concrete. The slab has reinforcing mesh too since it is over different bits of concrete. BCO has approved my construction spec.
OFTEC never seem to consider in their diagrams fire walls. Having 300mm all round when the tank is in the middle of nowhere is clear enough but if you are building a fire wall then clearly you don't need the 300mm. OFTEC say somewhere allow 60mm for tank bulging so I've left at least 100mm clearance to the fire walls.
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