Howdies Kirsty...
Love your kitty on a leash, mine act like they are drunk and the leash weighs a thousand pounds.
Cring of cringes.... I sit on various chairs depending on where I lug my machine to.
I do have a sewing desk but don't like to be alone if my DH is home. It has a regular office chair in it that can be adjusted.
Other times I sit on a straight back kitchen chair. When my rear gets tired, I add a bed pillow to it (also gives me a different position for my arms/shoulders to be in).
I've heard you are suppose to adjust your chair height every so often to give your body a change of position and it helps to keep from having aches and pains.
Don't forget to relax your shoulders while you sew and keep good posture if possible.
Welcome to the board too.