I don't know exactly what an airing cupboard means, but if I understand the problem, then it is just about volumes of cylinders.

V = pi(r^2)h

r = (425mm)/2 = 212.5 mm = 21.25 cm. maximum

V = 115 liters = 115,000 cu.cm.

So,

V = pi(21.25^2)h = 115,000

h = 115,000 / (pi *(21.25^2)

h = 81.064 cm = 810.64 mm high or lower. -----------answer.