Hi there, thought that seemed small
AFAIK there is no regs (17th) with regard to space factor but after reading around apparently 15th edition quoted 45% which is what i'd go with as a minimum if I were in your position.
Just some scribbles but........
32mm diameter x 0.5, squared and multiplyed by pi = approx 804mm(squared) csa
40mm diameter x 0.5, squared and multiplyed by pi = approx 1256mm(squared) csa
8 x 804 = 6432 + 1256 = 7688mm(squared)
7688 / pi = approx 2447, sqrt of that is near as damn it 50, multiply this by 2 to give total diameter (of cables and insulation) = 100mm I would multiply this by 2 to allow for the gaps between each cable and then multiply this by 2.2 to allow space factor so this gives
440 mm diameter which is 17.3"
Whatdya reckon? I have no idea if this is ok BTW