Finally got the sofware doing its thing and got the answers I need...

Worst case scenarios, fully loaded with 4.5tonnes of wall/roof, live load and mill etc... and including 8.8tonnes of self weight!

Clay shrinks under hard core in centre and theres only support at corners (centre sags 3mm)

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Clay heaves under hard core and lifts slab off footings (corners drop about 2mm)

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Clay shrinks at one end and leaves end floating in space.... (it drops 7mm or so)

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Using BS8110 part 3 tables I now know that i need some substantial mesh/reinforcement...

In slab I need B1131 (100mm/12mm dia x 200mm/8mm dia)
In beams I need 10 x T12mm bars, 5 top and 5 bottom on the long side and 8 x T8 (4 top, 4 bottom) on the short sides.

Its cheaper to use the more expensive mesh than to make the concrete thicker...