A core assembly comprising: a core including an exterior surface (124; 224; 324; 424; 524; 624; 724; 824; 924) having a recessed area (128; 228; 328; 428; 528; 628a, 628b; 728; 828) extending along the exterior surface (124;...924), the core comprising: an upstream end (120; 220; 320; 420; 520; 620; 720; 820); a downstream end (122; 222; 322; 422; 522; 622; 722; 822); and a second side (126; 226; 326; 426; 526; 626; 726; 826; 926) extending between the upstream end (120;...820) and the downstream end (122;...822); wherein the first side (124; 224; 324; 424; 524; 624; 724; 824; 924) extends between the upstream end (120;...820) and downstream end (122;...822); and an insert (108; 208; 308; 408; 508; 608a, 608b, 708; 808; 908) including a contact surface (148; 240; 342; 440; 540; 638a, 638b; 738; 840) corresponding to the recessed area (128;... 828), wherein the insert is a refractory metal core including a first end (140; 230; 330; 430; 530; 630a, 630b; 730; 830; 930), a second end (142; 232; 332; 432; 532; 632a, 632b; 732; 832; 932), a first side (144; 234; 334; 434; 534; 634a, 634b; 734; 834; 934), and a second side (146; 236; 336; 436; 536; 636a, 636b, 736; 836; 936); wherein the contact surface (148;...840) is located on the first side (144;...934) of the refractory metal core (108;...908); and wherein the recessed area (128;...828) is located on at least one of the first side (124;...924) and the second side (126;...926) of the core; characterised in that the first end (140;...930) of the refractory metal core (108;...908) is located adjacent the downstream end (122;...822) of the core. a core including an exterior surface (124; 224; 324; 424; 524; 624; 724; 824; 924) having a recessed area (128; 228; 328; 428; 528; 628a, 628b; 728; 828) extending along the exterior surface (124;...924), the core comprising: an upstream end (120; 220; 320; 420; 520; 620; 720; 820); a downstream end (122; 222; 322; 422; 522; 622; 722; 822); and a second side (126; 226; 326; 426; 526; 626; 726; 826; 926) extending between the upstream end (120;...820) and the downstream end (122;...822); wherein the first side (124; 224; 324; 424; 524; 624; 724; 824; 924) extends between the upstream end (120;...820) and downstream end (122;...822); and an upstream end (120; 220; 320; 420; 520; 620; 720; 820); a downstream end (122; 222; 322; 422; 522; 622; 722; 822); and a second side (126; 226; 326; 426; 526; 626; 726; 826; 926) extending between the upstream end (120;...820) and the downstream end (122;...822); wherein the first side (124; 224; 324; 424; 524; 624; 724; 824; 924) extends between the upstream end (120;...820) and downstream end (122;...822); and an insert (108; 208; 308; 408; 508; 608a, 608b, 708; 808; 908) including a contact surface (148; 240; 342; 440; 540; 638a, 638b; 738; 840) corresponding to the recessed area (128;... 828), wherein the insert is a refractory metal core including a first end (140; 230; 330; 430; 530; 630a, 630b; 730; 830; 930), a second end (142; 232; 332; 432; 532; 632a, 632b; 732; 832; 932), a first side (144; 234; 334; 434; 534; 634a, 634b; 734; 834; 934), and a second side (146; 236; 336; 436; 536; 636a, 636b, 736; 836; 936); wherein the contact surface (148;...840) is located on the first side (144;...934) of the refractory metal core (108;...908); and wherein the recessed area (128;...828) is located on at least one of the first side (124;...924) and the second side (126;...926) of the core; characterised in that the first end (140;...930) of the refractory metal core (108;...908) is located adjacent the downstream end (122;...822) of the core.