Ok so 1000L = 1m3 , a VERY rough calculation assuming the...

Ok so 1000L = 1m3 , a VERY rough calculation assuming the reservoir was a 500m diameter circle, would give close to 200,000 square metres. Divide 5 mil by 200k, 25 metres in head height. That is a lot more than I would have expected.
That only equates to about 250kpa but the constantly changing pressure in both reservoirs will make optimising pump efficiency more complex.
Happy to be proven wrong if I have missed something... it's Saturday!