I've given up trying to second guess what planners will do, I'm usually wrong.
It probably wouldn't do any harm to submit a new prior notification larger home extension application but don't build your hopes up. Maybe change the layout slightly so instead of being 6m deep make it 5.8m or something and claim you are resubmitting because of the design change.
Otherwise like I said originally, you will have to submit a new householder planning application. I have no idea how they will feel about a 6m deep extension, that is quite big, so you might have to make some enquiries, most planning departments will not give free pre-application advice anymore so a GOOD local architectural designer might be a better source of advice. Your existing "architect" sounds like a bit of a numpty but he might be able to give you some advice on likelihood of planning approval and will probably be cheaper as he already has the plans prepared.
