Shower is dragging water out of bathroom sink trap.

Why has it started doing this in the last few weeks? The trap gurgles as you shower and then the next time you enter the bathroom it stinks of sewers. It never did it before.

The soil pipe looks to be clear at the top vent, which has a good cover on it. The shower trap is a low profile "waterfall" type. The sink is actually in an adjoining bathroom but must be no more than three feet away from the shower trap, which I think joins the bathroom drain just downstream of the sink waste and the drain runs at a shallowish angle under the length of the bath and out of the wall to join the soil pipe below first floor level.

Any suggestions?

If the basin waste has an anti-vacuum valve on it, which would be advised given the configuration you have, it may have become stuck in the closed position. If there is one, try giving it a clean.

Anything slow to drain? You might have something pulling the trap.

More likely to be a restriction in the waste somewhere, the pipe will then fill with water causing a vacuum behind it, it will then pull on the nearest trap on the same run.

Well I cleaned all the hairs out of the shower trap last night - disgusting job - and poured some bleach down the bath and sink wastes and this morning the bathroom smelled okay, so I don't know. It all appears to be flowing ok with no blockages.