As gmartine says, really. You'd get away with chipboard or OSB. But please don't. Realistically, tongue and grooved boards are fine, and any plywood should be fine. The point of WBP is it will stand repeated condensation and drying cycles without the glue that holds it all together failing. I suppose if people really used their brains, the cheapest chipboard, thorougly coated with bitumen and then covered by a 10mm sheet of Kingspan would be seen as acceptable, but we write laws to cover situations where people don't comply with the law in other areas, so it all gets a bit silly.
For instance, what most of the press did not bother to tell us is that the baby,
BBC NEWS | UK | England | Somerset | Focus on baby death safety checks - http://news.bbc.co.uk/2/hi/uk_news/england/somerset/7179092.stm that was drowned/scalded to death follwing a faulty immersion heater thermostat overheating a cistern, might have escaped death if the property had had a properly installed loft cistern in the first place. According to the BBC, 'the inquest was told the tank at the Hardies' home had been mounted on an old door during modernisation work in 1973 and part of the tank was overhanging.' So what do we do as a nation? Send inspectors to check the Rubbish that has gone on in council housing and the private sector and ensure that landlords fulfill a duty of care in this and in other matters (which would also have prevented Grenfell happening)? No, instead we insist that all new thermostats be fitted with a safety cutout and we change thermostats in council housing. Which saves a death every decade or so.
I'm convinced 100mm is a law and that so is marine ply, but even I struggle to see under what conditions a 75mm underlap wouldn't be enough, and it seems sensible to think that a single piece spanning all joists is better than 100mm underlap and have to cut it in half, thus spreading the weight less effectively. Am I to believe that a metal or asbestos-cement tank needs a 100mm underlap to be safe? What if it is literally only ever used for cold water and not connected to a cylinder at all? Yet I don't think the law specifies the cistern material or temperature application. So, I'd also suggest a circular and high cistern needs more of a support than a long and low cistern, and I'd particularly doubt a coffin-type that has only two small access holes in the solid lid would be likely to split whether it has a 100mm underlap or a 10mm one. Probably the 100mm rule is there because if the cistern gets shifted, if has to move 100mm before it starts to fall off the edge. Use your common sense - the fact that you are bothering to ask this sort of question rather than go ahead and say 'Stop bothering me! - I know what I'm doing... Leave me alone! - I've never done this before' suggests you are unlikely to get it far wrong.
If I were in your situation, I'd consider 75mm will be fine, particularly if I had overheat devices built into my boiler and, a cut-out on the immersion heater thermostat, and made sure the cistern is dead central and stays dead central. Then you'll die when a volcano erupts directly below your house - hadn't thought of that, had you?
