I have one main contact at Liverpool tower, I have his mobile number and I ring or text when I have a friend/family interested. It's a good excuse for me to go again.
The chap at LPL usually arranges for me/us to visit at the end of his shift, so when he finishes at, say, 2pm, I can go along and spend an hour or so there, they do welcome visits it seems.
As I said, I choose to take a token of appreciation, after all, this man is giving up his own time to show us around. (Personally, I can't wait to scarper out the gate when I finish work
)