I think if I was you Jenny, I'd like to have more information about the purpose of their survey and to talk to them about the points you've raised with us.
I would be curious to find out if there is perhaps something bordering your land that has lead them to wanting to do a survey.
Talk to them about it. I'm sure their intentions are favourable to you, but there's nothing like a face to face discussion about your concerns.
Perhaps Gill will be able to fill you in on what it would involve-if she doesn't mind that is.
Jules