We think there are two factors causing this; the grate bars are a little too close together to allow the ash to fall through into the ash pan, so it remains in the firebox to get melted and form clinker, and the fire bars run across the firebox making 'raking through' the ash with the dart less effective than if the fire bars ran front to rear of the box.
I therefore ordered a new grate which we have trimmed to the same size of the original, to address both these shortcomings.
Wildfowler's dampers open (there's one on the other side of the ash pan as well, both controlled by a single lever in the cab). When a retaining pin is removed, the grate drops down on a hinged cradle into the ash pan, and can be withdrawn through the damper opening.
The new grate halfway in to the ash pan through the damper opening. Once the grate is fully inserted, the grate cradle can be raised manually to locate the grate in the base of the firebox, and the pin inserted to retain it there.
The new grate on the left, the original on the right. Note the fire bars run front - rear on the new grate, side to side on the original. The fire bar spacing is wider on the new one, though that is exaggerated in this picture by the angle it was taken.
The damper in the closed position
The lever on the extreme right controls the dampers
Next time I run Wildfowler it will be with the new grate. I'll let you know how we get on.