There are two little brackets that sit behind the bumper one at the rear face and one at the top. I assume its the one at the top you are missing, the 3 clips on the winglet clip into it.
The part numbers are
RH Upper 57743FE040
LH upper 57743FE050
RH SIDE 57743FE020
LH SIDE 57743FE030
Just for info the fog covers and winglets are not that expensive from the dealer and come pre-painted. The part numbers are below, prices may be a bit out of date but worth getting a quote before going for pattern parts.
STI Fog Lamp Covers 57731FE300TG (TG code for Silver) Price £34.17 plus vat
STI Fog Lamp Covers 57731FE310TG Price £34.17 plus vat
Aero Trims (Winglets) LH 57742FE010TG Price £57.72 plus vat
Aero Trims (Winglets) RH 57742FE000TG Price £57.72 plus vat
Bolts X6 901000186 Price 0.25
M6 captive nuts X6 901000186 Price 0.25
Hope that helps