Will a 3 phase inverter fitted to a shop affect the flat above?

If your electrician installs trhe inverter correctly and tests it, and issues a test certificate then there will nbe no problems. If you are doing as a DIY job, then there is a potential to get it wrong if your not expereienced with 3 phase and connecting inverters. Under normal circumstances it will not affect the flat above teh shop in any way as they should have a single pahse meter for their electricity account and the shop sghould already be separate.

Not unless you're stealing power from the flat above to run your inverter ;) Seriously though - I'm not sure what your concerns might be. In what way do you think it might affect them? As long as the (presumably) dc supply feeding your inverter and the 3-phase output from it are isolated from the upstairs circuits, and it doesn't make too much noise, then - notwithstanding all relevant electrical regulations - I'm not sure I see any problems. Or am I missing something?