MTB to Single speed commuter?

[uncinquesei]

As we head into the winter proper, I'm thinking about what to do with my old mountain bike... It was a lovely thing when it was new (2001) - Marin Nail Trail but it's done a lot of hard miles. I love riding my single speed for commuting - simplicity, love the challenge, fitness etc. and was thinking maybe I could convert the MTB. The gears get a hammering on my ride; 50% of it is sea front so a lot of sand gets into everything, 50% is cycle track, lots of leaves, wet muck etc so a perfect ride for creating and applying your own grinding paste.
If I were to convert the MTB, what would recommendations for gearing etc be? Current single speed (self built 1970s BSA) is running 48:18 on 700 / 23 tyres. MTB is on 27" wheels and would be used for the really sh£$%y days.
Recommendations?