I don't think it's really a DIY job - roofs are very high
we used to have 5 chimney pots (Victorian house) - we had builders in for various jobs & they took 4 of them down from the roof & capped off the holes (I think they put a curved thing on top to let some air in to ventilate the flue)
we still have one of the tall pots in the back yard with a planter in
I've called up two chimney sweep companies and one has quoted me £80, and the other £50 (with a sweep and smoke certificate included).. I've managed to convince DH that he shouldn't attempt it himself! Hurrah!