DS5 door open issue - won't lock

[The Negotiator]

Dear All

My wife has a 2015 DS5 HDI DSport which has handsfree locking. Over the last day or two, the car has on occasion refused to lock as if one of the doors has been left open, each time this has eventually resolved itself after a bit of waiting but it's obvious a problem if/when it occurs again. Whilst driving she noted the warning "rear right door open" so I am guessing that this is the fault stopping the doors from locking and that it is intermittent.

Is anybody able to advise on the most likely cause of this? I was reading that there are different modules depending on whether the car is handsfree locking or not but couldn't work out which item would be needed in either example. I can see some of the door locking modules on ebay but am not sure if this is the right thing to replace. I also see possible issues with faulty wiring on the forum.

Thanks in advance for any help.