Replacing concrete around drain for new patio

[GMAD]

I'm currently ripping up our our horrid slabs and rubble base (laid by previous owners 20 yrs ago) to replace with Type 1, sand and sandstone. In one corner we have this 25+mm slab of concrete surrounding a drain (w/m waste). What's the best way to remove this without damaging the drain? Have a kango but unsure if this is too violent, hence wondering if a concrete cutter might be better?

The slab has a slight fall that gets most rainfall into the drain. The patio itself will fall away from the house.

Any ideas?

TIA