Bates are really good, they aren't cheap but they have a piece of webbing in the centre that stops the leathers stretching. I've had some for 4 years & they haven't stretched yet.
whatever you do... dont buy the same shires ones as my mum! she bought them for me... I used to ride 3 from top, I now ride top after 4 months? Soon be twisting or punching
I have some similar to Bates which have nylon in the middle - don't know the name of them but same so can recommend. I bought a pair of shires which were an odd length !
I have jaguar ones. Lovely, and the buckles are angled so they don't dig in. Have a nylon core and the best bit is that the holes are close together. Pricey, but well worth it.
My Stubbens haven't stretched at all and I've had them a few years, they're also easy to change the length on. Just bought another pair and they're just as good.
I've had my Treadstone leathers for 4 years and no stretch, you just need to go for ones with a nylon core to stop the leather stretching or really good leather ones eg stubben (as these should be tanned in a way that they won't stretch)
I've been a bit disappointed with my wintec pro webbers, as they are falling apart in under a year.
I have amerigo ones which are lovely, and bates normal ones, which aren't quite as nice. My pessoa ones are looking a bit sad now, but I think that's more the silly tan colour.
As mentioned about Cotswoldsport (moi) have some, but they turned out more like buffalo polo ones. So uber strong, but thick with it!
I have Devoucoux ones that are 5 years old and still the same length as the day they arrived, they also have the holes closer together which is great. Exselle do good ones too and are cheaper than the devoucoux.
I have the plastic thorowgood ones, they don't stretch because they have a web core, however they do look like something off Barbie's show jumper set and they are a pig to adjust when you are mounted. Plus point is they are cheap and don;t need to be cleaned and they won't break