Down pipe coming off canopy fitted needs to be somehow fitted to existing downpipe

It's hard to tell without seeing the job, but if your cast pipe goes directly into the ground without a visible surface drain you will need to put a Y branch into the cast pipe. Cutting these cast pipes is not easy requiring an accurate cut with an angle grinder. Your safest bet would be to get a professional to do it for you.

This would be very difficult to cut out a cast iron pipe to fit a branch as cast iron pipes are fitted to wall securely and no movement on the down pipe to fit a branch and can cause more damage to brickwork render or actual pipe trying to remove it I would personally cut off pipe at ground level and attach new cast iron down pipe or plastic therefore branch would be fitted correctly this would be up to individual to decide which they would prefer

You can get a downpipe connecter from any good building suppliers and it will fit straight in, you can get square, round and square to round.

On a professional view unless your planning on renewing old cast iron, personally I’d take rest of exciting cast iron off and replace with complete plastic as regulations would recommend