Thanks for the update OP!
I would 'maintain a presence'... ie, visit the allotment several times, if she IS there, be polite and move along unless you have some specific task to do (measuring shit, or actual dimensions of things rather than just shit), but just subtly apply a weeee bit of pressure, say hi to other allotment users if they are there. Be sunshine and light at all times of course.
And yes, take photos of EVERYTHING.
That way (asides from providing us with updates on which way her deckchair is pointing and how she glared at you), you're making your faces known there, making connections with others there and letting her know that this isn't going away. In my experience, some people can easily shove a letter in a bin or in the back of a drawer and 'lalalalala it's not happening' otherwise.