I made my comment mostly as general information. There are many with similar problems who follow threads of this type. As for the load-bearing capacity of your bjälklag, I dare not comment since I haven't seen any information on beam dimensions. I have some doubts since it's very heavy stuff you want to place on top.
Okay, I'm with you. 
The beams in the floor structure are 180x250mm (hxb) timber logs (roughly the same on all. The CC measurement is up to 800mm in some places but about "standard" 600-700mm. The risk is perhaps that the cc measurement is too large and that it doesn't help at all with nogging at cc 300?
The beams in the floor structure are 180x250mm (hxb) timber logs (roughly the same on all. The CC measurement is up to 800mm in some places but about "standard" 600-700mm. The risk is perhaps that the cc measurement is too large and that it doesn't help at all with nogging at cc 300?Yes, those are sturdy structures! You can drive a tank on top. In addition to the useful load and the basic weight of the framework, you can load almost 2000 kg/sqm in the form of EPS concrete and leveling compound, etc. Joists do not help with stability. You do not need to correct where the c/c is 800 mm.
That's good, it was reassuring to hear! I'll go for it then!