Has anyone got any tips for the holy grail of achieving the amazing tender lamb and beef of the BIRs of old?

Many recipes I see say eg use chuck or stewing steak for curries as if it doesn't matter - I've never found that, if you start with tough meat, you'll end up with the same. Only tried the slow cooker a couple of times but was disappointed there also.

So what's best, do you just get the best you can afford or would a Halal butcher be a better option?

This was a real favourite in the Glasgow curry houses of old. I tried to recreate it last week using diced lamb from the supermarket and fresh fenugreek (methi) leaves, done for 5 hours in the slow cooker. Two issues - the lamb was not tender, also the leaves didn't seem to break down and impart the same colour or flavour to the dish.

Does anyone have an authentic recipe, also what type of lamb is best, should you use halal? Would dried leaves be better than fresh?