The requirement is for a "bunded area capable of containing 110% of the storage capacity of the facility.

In my workshop, I had the floor sloped to a sealed sump in the floor. I calculate that it will hold over 200 L in the event of a spill. I have had some small spills which did not make it to the sump, but the issue which gave me grief is when the rainwater tank overflowed during a heavy downpour and the water banked up and came into the personnel door of the workshop. The bund was full and all of my cardboard and newspaper (used for making papier mache fire bricks) was saturated. Took some time to pump out and dry it all again.
I have since made a diversion so that any overflow water cannot come into the workshop.

Regards,
Tony