I managed to catch an adult moorhen this morning at Arnot Hill Park which completes the progression through the birds moult as adults undergo a complete post-breeding moult finishing in October/November. The underparts are then ash-grey, fringed white, especially on the belly.
The head, chin and nape are ash-grey and the mantle and back olivaceous-brown with a sheen to the feathers. The iris is deep crimson, they have a red band on the tibia and the 9th primary is broad and rounded (moorhen have 10 plus 1 vestigal primary feathers).
The most obvious difference of course is the red and yellow bill. This bird couldn't be sexed on its wing length of 176mm.