NEC3 PSC: Under Option E, how is price assessed if actual cost differs significantly from original estimate through consultant down inefficency, i.e. works taken too long?

Under option E the Consultant carries little risk. If the Consultant gave an estimate at tender stage then that does not contractually count for anything. I see no where in the cost reimbursable contracts that allows you to not pay the money the Consultant has spent. Option E contracts do require more effort and involvement on the Employers part to verify that the work being carried out by the Consultant is timely and to the required quality.