robgraham wrote:
I'm adding a shed on the side of my garage.

It works out that the new concrete floor level will just nicely line
up with the top of one of the existing concrete steps into the garage
- makes a nice reference point for the levelling.

However the existing step has quite a 45 deg. bevel on it's leading
edge. Do I need to treat this in any way to get the new concrete to
bond to it ? My concern is that if the bond isn't reasonable, the new
concrete will break away in time as it is potentially a weak spot.

A 45 degree bevel on a step? - are you sure it's not just a kerbstone laid
flat on it's back?
Either way, you can't 'chamfer' concrete in this way, you are right to
assume that it will crack up, the only way to stop this is to give it more
depth, IE cut out a straight line with a grinder and chisel off an inch or
two in depth, treat with neat PVA prior to concreting and scrape some slop
into it.