heater in transit swb 80 s reg works intermitantly?

the heater in my van transit swb 80 s reg only works for a few minutes then cuts out. it then returns to cold air for 10 mins then at some random point later it pumps out hot air,again for a short time. do you think its a mecanical prob or has a hose just fallen off (cant see one that has). How do i fix it as i am not very vehicle mechnically mined. as its below minus 5 at the moment with a foot of snow forecast need help asap plse.