I haven't grown howden before but it is a pepo type and I've grown pepos. Most start out green. Maxima types are usually yellow to begin with. But last year I grew two big max (maxima) vines - one had green fruit and the other had yellow fruit (initally) and they both turned out looking exactly alike - large red pumpkins. Bottom line - it is confusing - but I would guess you have a maxima fruit on there - either a squash or pumpkin. The shape shouldn't change much through the growing phase so that might give a clue as to what you have.