I have only ever used Yum Yum’s to order in from - so can’t really comment about the service in the shop.
Yum Yum’s have three menus really - a standard cold one (sandwiches, wraps, etc), a standard hot one (hot sandwiches, burgers, etc) and a specials menu, which is emailed around each morning around 9am. I normally ordered off the specials menu - not because the hot and cold ones were rubbish, but Fred’s (I assume the owner!) little comments about each item were just great and made you want your lunch before half 9.
(Fred’s comments were often the highlight of the first hour at work!)
The food was always very good from there, although slightly more expensive (iirc) than their competitors.
Yum Yum’s also do amazing sausage butties, perfect for a Friday morning (or any morning after the night before!)