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.