I've just built up my first road bike and was using my trusty park cable cutter on cable brake outer for the first time. The cut is very poor and resulted in the metal part of the housing cut badly so it added alot of drag. Is there something I'm not doing right or are my cutters sh** or shagged?

you need to snip it really quickly and if the outer goes oval just squish it round again with some pliers

burrs can be removed with a tiny allen key inserted.

Trim back the coiled steel part until it doesn't foul the nylon liner, then insert an old spoke to re-shape the liner.

If you bend the brake outer as you cut, it tends to open out a bit and make for a cleaner cut.

Always have a bit of inner inside when you cut. Not the new one though.

as BigJohn says, if you want a clean cut of the outer cable, make sure you have an inner cable at the point you want to cut it

With my bikehut ones, if it ovalises, use the flat section inside the cutting blades to reshape the oval. Then use a pointy thing to open the plastic sheath. I imagine nthe same can be acomplished with any good set of cable cutters.