I'm not sure sacking Allardyce now is the answer. Three permanent managers in a season is dicey territory. Plus, I'd rather save that money for the summer because this squad badly needs upgrading.
Sack Walsh now*, get his replacement in to start preparing for the summer, limp over the line, give Allardyce the axe in May.
* If the people in charge really do like him (I can't imagine why, but okay)... just demote him back to a scouting role. That's what he's supposed to be good at.
Edit: Assuming it's easier to sack him in May.