I have read lots of answers about dealing with the smell of drains from a washing machine, sink etc...
My washing machine plumbing seems unusual with the drain away pipe going into a long white pipe that goes under the floorboards but also goes straight up for about 50cms to an end point that has a kind of cap sitting on it. The cap could be forced down but I am worried that if I do this I may create too much pressure if drain gas is a problem.

Occasionally we get a smell of drains that seems to emanate from this pipe but at other times seems to be coming up through the floorboards in the hall about 2 metres away from the machine. The smell doesn't seem to be linked to any use of any specific thing, i.e. shower, washing machine, sink. Neither does it seem to be connected to hot weather, cold, rain etc... Sometimes it doesn't smell for weeks, then it just reappears for a day, or a month.

My questions are: what kind of sink trap or vent is this that ends with a vented cap on top - if I know I can check if it is installed correctly?

Do you think such an unpredictable smell can be from the washine machine vent - if so any ideas how to fix it?

I think you are describing a Studor vent or air admittance vent (AAV). It is a mechanical valve that should allow ait into the drain but not let sewer gas out, it replaces a stack vent to your roof. Since it is mechcanical it can fail but they are fairly inexpensive. I would not try to fix it, just replace it for about $8.

Thank you Ballengerb1 - I have looked up a Studor vent on the net and it looks as if that is it - I will get it replaced. Cheers

Identified the vent I described and gave clear practical advice

I cheated on you Melluton. I sneaked back into my post and added a 1 in front of the $8 cost. The AAV for your piping is more around $18 not 8. Sorry, fat little fingers and a very small keyboard. Its still a good price. Have a good one.