Dental floss is exactly the same as "Artificial Sinew", just a bit lighter grade and more minty.
If you are indeed in Sunderland, then taking a trip to Leprevo is
by far your best and most economical bet. Tandy are overpriced in any case, but having Leprevo on the doorstep is like opening the back of the wardrobe and wandering round Narnia in "tinkering with leather" terms.
For a start you can rat around in the "scrap box", where if you hit lucky, you can find top grade veg tanned off cuts, potential belt straps and thin stuff ideal for bag pouches and the like. Then , if you ask for Jez or Pete, you can ask for a basic starters kit-proper needles, one packet of 25, an awl blade and handle ( get at least two blades, you
will break the first one quite soon after starting!) some 18/3 linen thread-50 gms will do fine, if you dont already have a metal ruler, get a "safety rule" from LP, its got a profile like an "M" but a bit more curvey and they're very good indeed for safe cutting as well as marking lines etc. a block of beeswax for the thread is good and if you don't have one, a decent quality craft knife or scalpel and blades.
The hole punch is a good idea to start, only if you intend to make belts or items that require a significant number of proper holes.
Use a dining fork, not only for marking your stitch holes, but also to actually mark the sewing groove. It's very easy to do but not so easy to explain
BEST and by far the most worthwhile thing to buy if you go there is: Al Stohlmans, "The Art Of Hand Sewing Leather" the best £10 or so, you will
ever spend in connection with leather working.
so;
needles
awl
thread
ruler
wax
scalpel
book
and pinch a dining fork!
Good luck and if you get stuck, drop me a pm.
Steve