Plumber recommendations for suspected leak in Edinburgh tenament

Just bought a flat in an old tenament this year and we've been getting complaints from the shop downstairs about leaking coming in through the ceiling. It is not constant and appears to be intermittent but seems to roughly coincide with us doing laundry.

Have had Haggart Plumbers out twice but they don't think it is coming from our flat. The last time I turned on every appliance including the washing machine for 30 mins while the plumber waited downstairs with a step ladder but he could not identify any leaks coming from the flat.

My worry is that the issue only happens when we have longer laundry cycles. Would anyone be able to recommend a plumber who has helped out in a similar situation?

Being a first time buyer has been stressful enough without this but want to get it sorted.