If you mean the lushprojects diagram, Y goes to the top of the radiator, Z goes from the bottom of the radiator, X goes to the bottom of the thermostat on the passenger side of the engine & 15-186 goes to the top of the engine on the drivers side - this is the one that started this saga

Check the link that Ian posted to the club shop, prices a bit better than Bongobits if you're a member, the main offending hose is the 4th one down on his listing, the three above it are the three others mentioned above.
It may be as well to replace the hose that goes over the starter motor to the rear heater from the engine too, Ian or Bongobits or Flippa (ebay silicone hoses) will know what you mean, also while you're at it, it's probably be worth changing the stat & make sure he does a proper flush - preferably with the stat out ( just make sure it goes back in

) & backflush the rad.
Make sure he gives you some of the same coolant to top up with, should you need to & I would highly recommend getting a Mason or Coolant alarm fitted if you don't already.
You can't polish a turd - but you can roll it in glitter.