The MCA have recently formalised an arrangement with the PYA and Nautilus for them to verify all sea service before it is submitted with an NoE application.
You do not need to be a member of either organisation to have your service verified, but there may be a charge for this service for non-members.
MIN 543 details the requirements, but failure to have your service verified by one of these organisations prior to applying for your NoE will cause severe processing delays of around 160 days.
The service requirements are relevant to all routes laid out in:
MSN 1858 - Deck Officers on Large Yachts;
MSN 1859 - Engineer Officers on Large Yachts;
MIN 524 - Small Vessel Engineer Officers (Yacht applicants only);
MSN 1861 - Revalidating CoCs.