GB wrote:

https://i.ebayimg.com/images/g/EzcAA...7j/s-l1600.jpg

Bosch still sell these, but annoyingly they charge around £25 for a set
of 4.

Can't you source some other O-rings, based on the dimensions?

https://www.cyberpieces.com/25457-large_default/joint-de-l-adoucisseur-d-eau-bosch-00166878.jpg