Lancasters had different propellers. Some having the paddle wheel props, and others having (I forget the proper name) the pointy props. Anyone know how or why there are two (or more) types of propeller, were they both used indiscriminately on all Mks ??

Ithink but not sure that as many non aeronautics manufactories built lancs frame to help war effort, the props were set following stock deliveries contract and not for any technical reasons. As i told you i'm not sure so if any has other explanation there are welcome.
BAT.21

My guess would be that, when one type was broken, they just used the other if they had nothing else around. That would make some sense.

~Chip :-)

I have seen photos of MkIs and Mk IIIs with the needle and paddle prop ( the reason for my question) but not both props on 1 aircraft. If one prop was broken, they would change all four ??

Of course just for a simple reason of output.
the shape of blade being different the power delivered by rotation is different.
This is the same as if you had 2 different wing profile on same aircraft; for sure you will encounter problems of flight.
Bat-21

One reason would be that the needle prop is more aerodynamic at lower altitude, while the paddle-blade prop is more effective in the thinner air at higher altitudes---the same reason we put paddle-blade props on our high altitude FlyingForts and SuperForts. As German Flak directors and night-fighters improved, the RAF increased the flight level of its bomber streams and designated release altitudes over targets in an attempt to get out of their effective ranges

Could the diferent propellers also be to do with the power output of the engines? I once read that the spitfire Mk9 was fitted with a 4 blade propeller to absorb the greater power of the latest merlin engine. The merlins used on Lancasters did vary to some degree. Could this be the reason for the wider paddle shaped prop blades? Just a theory