I'm in the process of installing an Ideal Isar 30HE boiler, and the manual
says if I'm connecting the condensate pipework to a soil stack (which I will
be doing) that I need to fit a 75mm trap.

The questions are.

Do I fit the trap inside the house or outside, or doesn't it matter.

Where can I buy a 75mm trap to fit standard 21.5mm plastic overflow pipe
(I've googled, froogled and poogled till blood comes out of my ears, but no
luck), or do I buy a 32mm trap and then try and find 21.5mm to 32mm
reducers.

Or don't I really need one (hoping this is the answer)

Cheers

Jon