Hi kt17, and thank you for the reply. I am new to this site and, of course, now realise it was silly of me not to add the info!
Yes, it is indeed the Royal Fleet Auxilliary, and the answer has six letters, the second being E, the fourth being S.
There are both Bay class and Wave class vessels in the fleet, and my first thought was that FAL was similar, for example to "LSL" and acronym for Landing Ships Logistics, also a class of vessel in the RFA, and that WAVE would be an acronym for another class of vessel.
However, while I could get "Auxilliary Landing" from FAL, I can't determine the F, and for the life of me, I can not come up with anything related to WAVE.