Immediately distracted by my passengers and probably because closing flight plans isn't normally something I have other think about.  It was 10 minutes later when I was asked how I'd closed my flight plan by another English pilot, also having trouble with the french phone number, I remembered I'd still not done it!   Yet another English pilot then suggested I could use SD (this was new to me) or call one of the lesser known regional numbers for flight plan closure.  

I decided to use SD and then check (big fine for unnecessary S&R callout in France) with one of the regional numbers to see if the SD system had worked.  It had, the French ATC confirmed my FP had been closed.  

How easy would it be for SD to ask you if you would like to close your flight plan when perhaps you log "On Blocks" at the end of a flight, if SD can see you have a flight plan open?  It would take some of the human error / post landing ability to forget out of the situation.