I advise to double check size with the seller before committing. see:
https://www.ebay.it/itm/364735402883 Most of the 1940s/50s seat collars are by Magistroni, and /or branded Bianchi, Olmo, Ideor and Legnano. Search ebay.it and enter 'collarino morsetto regisella e'poca' to see other choices. Use Google translate option on the pages. Check out postage tab 'Spedizione'-not all will sell to UK, but the Bianchi seller has listed it for UK postage at 12 euros and the collar is 10.80 Euros. Good luck!