Two things soon became clear...
1) The high street 'chains' of coffee shops aren't interested in allowing struggling authors to leave promotional cards for their customers - whereas the smaller, independent coffee shops don't have a problem with it - especially if you buy a coffee or they've seen you before!
2) Sometimes you come across members of staff who are exceptionally helpful and who make your day - by treating you like a human being! This happened in Waterstones, Jameson Street, Hull when the young lady on the desk gave me the manager's number, as he wasn't in today, and then took a book and a card - all without making me feel like I was being a nuisance! (Even if they don't decide to stock my books... their staff are STILL extremely nice!) and also at Hull Central Library. There, the librarian at the help desk went off to find someone who could help me and then sought me out in the adjoining coffee shop to give me the details of the Head Librarian and take a stack of postcards off me! He really was 'a very nice man'!
So... tomorrow I have phone calls to make during my dinner hour and another week I'll be tackling York. I have high hopes for York - There are lots of independent coffee shops, and believe me... I'm a customer of quite a few! :)