The condensate from my combi boiler drains via a connection at the kitchen
sink waste. Whenever the sink is emptied (etc.) this produces a bubbling sound
from the boiler, which I'd guess is caused by suction acting on the condensate
reservoir/trap inside the boiler.

I'm not sure if it's doing any harm, but was thinking about the best way to
stop this happening, short of relocating the drain pipe. Suggestions please .