get a new circlip to hold the two parts of the speedo cable together?

Failing that get a good second hand replacement from a wrecker. No need to go straight to new. Although this is your second speedo cable IIRC they generally last ages.

Is your dump pipe heat shielded or wrapped? Speedo cables hate excess heat because the grease inside them dries up and the the plastic components become brittle.

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RS Liberty - Because WRX's are only good for parts...