Pete,
If you choose to retain your 10-mm banjo fuel line, I would use a coupler (12-mm x 1.5) from Belmetric to connect the FP check valve (12-mm x 1.5). First, this is mock-up how a FP-984 (with integrated check valve) connects with a 10-mm banjo fitting used in early SC CIS.
Shown below are the different components used with FP-984 to connect a 10-mm banjo fitting.
Assembled unit:
A typical Bosch FP without integrated check valve:
Assembled unit using a coupler or adaptor:
https://belmetric.com/stainless-steel-fine-additional-lengths-hex-coupler-standoff-nut/?sku=NLONG12X1.5X30SS&gclid=EAIaIQobChMI0LSyjfab-gIVVcvICh3pzQByEAQYBCABEgJrG_D_BwE
Tony