Philip
On Moderation
One thing I never quite understand with this 'Northern should run three trains a day to London' idea, and seemingly the corresponding aversion to advance tickets on Avanti is that any ticket bought for the Northern service effectively becomes 'booked train only by default, as there is only one option at the time any prospective passenger is likely to travel.
Part of the reason for 3 trains per day is that it'd keep the costs down and so potentially this would allow Northern to sell some very cheap tickets for the service, say £20 Northern-only SVR without restrictions and £7.50 Northern Advance singles, none of which would be possible if they introduced an all day service because of the increased cost.
If they were clever with timing, it could become almost like a people's service; offering people in Manchester & the North West in general the opportunity to have a day out in London at a very affordable price without having to change.
So for example southbound departures from Piccadilly, Oxford Road or Victoria at 06:30, 07:30 & 09:00 for late morning or lunchtime arrivals into London. Northbound departures from Euston at 17:00, 18:00 & 19:30. It'd allow passengers anything between 4 hours & 13 hours to spend in London. Avanti would continue to cater for the business market, Northern would be focusing on leisure.