Our "bratty sweetheart" is almost 3.  We got him from a breeder in Eden NC. (We live in Fairfax VA).  He looks amazingly like your Tenby!  His head was almost all black when we got him at 11 weeks old....reddened over time.  He's very smart...always knows when it's time to go to the bus stop or when the dishwasher is being loaded.  He' s lovable (wants to cuddle on the couch when it's Netflix movie time) but also very stubborn and definately has some bad habits.  He's very social with other dogs (has lots of neighborhood buddies and likes to get together with them) but I'm not sure he's mastered the social graces when he meets new dogs... barks at them loudly to try to say hello...they don't always want to say hello back!  Some of his bad habits...sigh... making a mess of his water and throwing his food bowl when he's hungry, stealing food off the edge of the table from unsuspecting kids,  stealing things to initiate a fun chase, lightly nipping at our thighs when he really wants something (yeah, this is a bad one we have to work on!).  He's definately better behaved when he gets the attention/stimulation he needs.  I'm trying to take him with me more when I go out.   He's definately all Corgi... highly highly motivated by food, won't come when he's called unless there's something in it for him, pushes limits!  At the same time, he wants to be part of everything social in the household...my husband will wrestle with my 10yo son and Tenby comes running to get in on the action.  When kids play basketball, he's the ref.

He's also very big!  I have him on a diet (he's 48 lbs), aiming to get to low 40s...he's just big. (Luckily he's very fond of carrots).

He has definately mellowed with age some and I'm hoping for more mellowing, but I know some of his bad habits, we just need to work on ourselves.  He has just trouble controlling his "enthusiam" for life. :) Has your Tenby mellowed with age?  I love the name Chepstow!