I actually just found out that Zipp are the only company who makes 100mm extenders,
100mm - https://www.tredz.co.uk/.Token-Valve-Lengthener_237305.htm? 100mm (slightly different design to what I prefer but suspect they'll still work pretty good having a proper core) - https://www.bikester.co.uk/kcnc-presta-core-valve-extenders-100mm-M938904.html 80mm so plus your 40mm valves would be 120mm - https://www.ebay.co.uk/itm/142709302560?
Thanks - I may well consider these if the Zipp ones don't work well!
@Charger started
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100mm - https://www.tredz.co.uk/.Token-Valve-Lengthener_237305.htm?
100mm (slightly different design to what I prefer but suspect they'll still work pretty good having a proper core) - https://www.bikester.co.uk/kcnc-presta-core-valve-extenders-100mm-M938904.html
80mm so plus your 40mm valves would be 120mm - https://www.ebay.co.uk/itm/142709302560?