Ok have received various messages from NA and Grey, this is their judgement
"Archie, the minimum ew bet is £250 ew and the minimum win bet is £500, so for an outlay of £800 it's not possible to mix both types. That leaves two possibilities, either £400 ew or £800 win.
DG is correct that £ 800 win is a different type of bet to the one originally intended, but an ew only bet is different as well. An ew bet is 50% win and 50% place, and a win bet is 100% win and 0% place, so your intended bet of 75% win and 25% is half way between the two. In the circumstances we think the logical thing is to let you choose between £400 ew or £800 win at the original price with Wiliam Hill of 40/1"