Marine Diesel Oil (MDO) and Gas Oil are two different products.Marine diesel (gas oil) sold on tidal waters is/should be FAME-free; that sold on non-tidal/inland waters will contain FAME.
MDO is a blend of gas oil and heavy fuel oil
Marine Gas Oil is a low sulphur FAME free fuel
Gas Oil aka Red Diesel can be MGO. It can also be, and frequently is, supplied from the same tanks as DERV fuel
The only refinery in the UK currently supplying MGO is Fawley and few marine outlets beyond the South Coast offer guaranteed FAME free MGO or MDO
Unless an outlet specifically states that it's fuel is FAME free it potentially sends probably isn't