Cable ties will be fine. The reservoir is surrounded by so may other bits of car that it will never fall out even if not held in with anything.....
The most important thing is that it is now all back together & everything works including the headlamp wash wipe. When I first switched on after re-assembling everything I still had the low screenwash error message. Happily this turned out to be correct & when I filled up with screenwash the message disappeared.
I need to revisit this. When I get a Low Screenwash message I can fill up with a few pints but previously I would fill until the neck of the filler pipe was full but now it starts overflowing from somewhere else on the reservoir before the neck is filled. If I fill up straight after the Low Screenwash message appears it's only about two pints that I can add before it starts overflowing. The neck is well sealed into the reservoir & it doesn't look like the leakage is from that joint. I assume that there is a split to hole somewhere else in the reservoir so I am going to have to remove it totally & replace it with the other reservoir.
I took another look today & I think I found the problem. The sensor & wire passes through a rubber grommet that should provide a seal but it has been pushed right in to the reservoir so that liquid leaks out through the hole. I need to dismantle it all again so that I can reach the sensor & re-seat it so that there is no leak.
I’m glad you found it, but I think it’s a bit of a job to get at.
I tackled this again yesterday. I had to remove the fog lamp to give me access then cut the cable ties securing the reservoir & was able to pull it loose enough that I could get at the sensor. The rubber grommet had either not been fully pressed home or had dislodged as I put it all back together lats time. I made sure that the rubber grommet was pressed firmly home & now I was able to fill the reservoir right to the top of the filler. I put it all back together held with new cable ties & now the jib is done at last.
On reflection I should probably just have shorted out the connection to get rid of the error message.