The battery seems low in cold weather, the engine turns slowly & fails to start

A slow CRANKING diesel engine has little chance of starting - especially in ‘cold’ temperatures!

Thats very helpful, cheers.

Jump start - replace battery, whatever - got to spin the engine fast enough so it will start ..

Yes will do. I think Ive not been doing enough milage, just geberally short trips, so battery hasnt been getting charged.