37d is thematic - 2 letter abbr. of a part of the UK (across the sea) + M(obile) - you need to replace the the first two letters with a 3 letter thematic item which i thought was an abbreviation but HotelWhiskey pointed out that it's the first part of an instrument C?? Anglais.
41a - 3 letters for "Mercury, say" (think Thor etc) reversed/"returns" + S(pecial) - definition is "men" (although I'd associate more with animals)
34d - Is also thematic (does that mean you've got one too many?) - original word is a 4 letter word meaning "vast amounts" around a single letter representing 1000 - then replace the 3rd and 4th letters with a thematic item.