Hi Howard,
The Renault ref for the mechanical speedometer (R21 L485) sensor is 77 01 464 631 but it seems to be not more offered for sale.
There is surely another way to find it. The espace 2 (renault code J638) has the same speedometer system.
As the housing for the differential has the same reference as the R21 (this means they are really the same), you could switch to components for the Espace 2.
Tachymeter mechanical sensor for Espace 2 is 77 01 458 336 and it is 59.09 euros (excl vat) and it is available.
Maybe the plastic crown would have to be changed too..In any case ref is 77 00 575 418 and price 8.40 euros excl VAT, so not a big issue.
I assume this would be ok as the differential housing is the same ref as the R21 and also the ref for the full gearbox housing (77 01 464 629).
Here is a copy screen of the internal Renault sofware for parts (espace 2 displayed).
Prices are maybe a little higher today, as I didn't updated my prices list since few months, no use for them in my case...You should add 5% to have an idea...
Cheers
Stephane