A 'nifty nabber' like the council guys use to pick up litter or the more difficult DIY version which is as follows..
A length of hollow tube (John Guest type stuff) push some string or wire up, then back down the tube leaving a small loop at the end - grip the finial in the loop and pull tight from the other end. Now you've got the damned thing gripped just got to get it back up there.
Oh! and to get the string through the hollow tube tie a pen to it n slide along - a friend told me that

and he told me he'd put mastic on the finial to stop the ****** idiot knocking it off again
