A wind turbine blade having a blade de-icing system, the wind turbine blade comprising a root end and a tip end, a leading edge and a trailing edge, the blade having an outboard portion provided towards said tip end and an inboard portion provided towards said root end, the blade de-icing system arranged to convey a heated fluid to provide heat to portions of the wind turbine blade, wherein the blade de-icing system comprises: an insulated outflow channel flowing from said root end to a location adjacent said tip end; a heating channel in the outboard portion of the blade, the heating channel flowing from said location adjacent said tip end along the leading edge of the blade in the outboard portion of blade; and an insulated return channel in the inboard portion of the blade, the insulated return channel extending from the root end side of the heating channel to the root end of the blade, wherein said de-icing system is arranged to convey heated fluid from said root end through said outflow channel to said tip end, from said tip end through said heating channel to said insulated return channel, and from said insulated return channel to said root end, said de-icing system configured to heat the leading edge of the blade in said outboard portion of the blade, wherein the wind turbine blade comprises a blade shell structure forming the outer surface of the wind turbine blade, characterized in that said heating channel comprises: an insulated wall (74, 96) arranged to define a leading edge cavity between said insulated wall and an internal surface of said blade shell structure at the leading edge of the blade in said outboard portion, and flexible seal members arranged between respective upper and lower sides of said insulated wall and respective adjacent portions of the internal surface of said blade shell structure, wherein said insulated wall (74, 96) is supported by an internal shear web of said wind turbine blade, wherein said flexible seal members are non-load carrying members, and act to isolate the forces associated with said insulated wall from the internal surface of said blade shell structure at said heating channel, and wherein the blade de-icing system further comprises at least one baffle (71, 74, 101, 103) provided in said heating channel, to re-direct a portion of a flow of heated fluid in said heating channel. an insulated outflow channel flowing from said root end to a location adjacent said tip end; a heating channel in the outboard portion of the blade, the heating channel flowing from said location adjacent said tip end along the leading edge of the blade in the outboard portion of blade; and an insulated return channel in the inboard portion of the blade, the insulated return channel extending from the root end side of the heating channel to the root end of the blade, wherein said de-icing system is arranged to convey heated fluid from said root end through said outflow channel to said tip end, from said tip end through said heating channel to said insulated return channel, and from said insulated return channel to said root end, said de-icing system configured to heat the leading edge of the blade in said outboard portion of the blade, wherein the wind turbine blade comprises a blade shell structure forming the outer surface of the wind turbine blade, characterized in that said heating channel comprises: an insulated wall (74, 96) arranged to define a leading edge cavity between said insulated wall and an internal surface of said blade shell structure at the leading edge of the blade in said outboard portion, and flexible seal members arranged between respective upper and lower sides of said insulated wall and respective adjacent portions of the internal surface of said blade shell structure, wherein said insulated wall (74, 96) is supported by an internal shear web of said wind turbine blade, wherein said flexible seal members are non-load carrying members, and act to isolate the forces associated with said insulated wall from the internal surface of said blade shell structure at said heating channel, and wherein the blade de-icing system further comprises at least one baffle (71, 74, 101, 103) provided in said heating channel, to re-direct a portion of a flow of heated fluid in said heating channel. an insulated wall (74, 96) arranged to define a leading edge cavity between said insulated wall and an internal surface of said blade shell structure at the leading edge of the blade in said outboard portion, and flexible seal members arranged between respective upper and lower sides of said insulated wall and respective adjacent portions of the internal surface of said blade shell structure, wherein said insulated wall (74, 96) is supported by an internal shear web of said wind turbine blade, wherein said flexible seal members are non-load carrying members, and act to isolate the forces associated with said insulated wall from the internal surface of said blade shell structure at said heating channel, and wherein the blade de-icing system further comprises at least one baffle (71, 74, 101, 103) provided in said heating channel, to re-direct a portion of a flow of heated fluid in said heating channel.