Were can I buy replacement knobs for my Halstead ace?

I recently replaced the pcb on my halstead ace boiler and need new knobs, I refuse to pay £7 + for a plastic bit, can anyone point me to a cheaper supplier. I require all 3, 1 large for the selection and the 2 smaller ones for heat settings plus the plastic clips that secure them in place.

any advice would be appreciated. cheap boiler, rip off spares prices