I don't know where in the UK, but you can get accessories directly from Sigma Sport. Their customer service should be able to give you your local dealer if you don't want to have the items shipped across the pond
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If the modern ones no longer have that function you could look for a used computer that fills your need. I’m happy to help further over the phone at https://www.6ya.com/expert/donald_f2ed37026a3ac881
Looks like that is a wired wheel sensor. Make sure the magnet lines up correctly with the wired sensor and is close enough to it. Also check for broken wires. If both those are OK and still no speed reading then the sensor is probably borked.
