Hmmm - I pasted this into the data validation and it's restricting my entries, even when they follow the correct format. I pasted this:
OR(MID(A2,9,1)="W",MID(A2,9,1)="H")*(LEN(A2)<16)+(MID(A2,9,1)="M")*(LEN(A2)=9))*(LEN(A2)=LEN(SUBSTITUTE(A2," ","")))*(ISNUMBER(LEFT(A2,8)+0))
And I tried...