Hi Jon, I was plagued with this issue during the last six months of the XF warranty.
The agents were good enough to show me how to drain the fuel / water separator,
which was a very easy job to do. (several times a week), It was obvious there was no water in the diesel.
Six months down the line, with the warranty just expired, they diagnosed a faulty sensor,
and replaced the filter unit, charging me £100+
Quite annoying !
In answer to your question, yes you can do it yourself.