I found a biodegradable penetrating oil based upon Castor Oil and citrus oil. Comments speak of it highly, especially for freeing stuck pistons and rings.
Called "S'OK", it is made by Shamrock Specialties, Inc. P.O. Box 1369 3503 West Oak Street Palestine, Texas 75802 USA
I've asked the manufacturers if it's available in the UK..
Regards,
Kim Siddorn