The list "British Tanks 1915 - 1919" compiled by Duncon Crow in "Armoured Fighting Vehicles of the World, Volume: one", 1998 Cannon Publications (previously Profile Publicatons 1970), says:
ive seen numbers, as 20 males and 30 females but since this wasnt meant for combat i saw 25/25 soudns better, combat mk's like the MK.IV MkI and Mk.V had more females than males
Roger Todd wrote: John Glanfield ('The Devil's Chariots') also backs up the 25/25 split and that, apparently, Foster's built the females, Metropolitan the males.
According to my source (New Vanguard British Mark I Tank 1916) exactly different: The 25 Male tanks came from Fosters, the Females (25) from Metropolitan.
The serial numbers were: Male tanks: 776-800 Female tanks: 576-600
Oops, you're right Hedi! Glanfield does say Foster's built the Males, Metro the Females - that'll teach me to actually read the source properly in future... Duh.