phew , TT agrees its a good mod 

The last time I applied castor changes was with adjustable arms on a old mini 
Showing your age, or just an enthusiast?

Castor effects self centering effort , am I safe to assume it will make the steering heavier in the off centre position?
Pedant mode - The effect of castor is
only felt during forward motion. "Swivel axis inclination" (the modern term for "king pin inclination" or KPI), works with the mass of the vehicle to provide a "static" self-centering effect, and this is then assisted by the castor.
Increases in castor angles will simply make the self-centering effect more pronounced, and
may be felt as heavier steering, though with the amounts from the Whiteline kit, I doubt it would be that noticeable at the steering wheel.
Dependent on the overall "dynamic" geometery, increases in castor can also increase the handling effects of the inner front roadwheels during hard cornering, though the only adverse affect is on bumpy corners, which will then give more "feedback" through the steering wheel - again, shouldn't be a problem with the small increases.
EDIT: on Reds corrections!
