Series III diesel starting problem

My 1982 Series III 2.25 diesel has a starting problem in cold weather. Previously it has always started after about 12 seconds on the heater plugs. Since the weather has got colder, I need to use the heater plugs for 12-25 seconds then try to start. Sometimes this works, but more often, the starter motor turns over very slowly (and sometimes clicks) and usually fails to start. By this time there's not enough power in the battery to start the thing. It works fine when jump started and usually bump starting also works. After a journey of 15mins then being left for 10mins this morning, it wouldn't start again (just a groan from the starter and a bit of clicking). A local garage has tested the battery, alternator and heater plugs and also investigated the fuel lines with no joy.
Any suggestions of what the problem may be, I'm getting desperate.

get another garage to check them for you..does it start with a jump start? does it start ok when its hot??

Thanks for the reply. Jump starts fine and usually starts okay when hot, but not always. Since I posted the message, the thing failed to start again. I managed to jump start it, but this time I discovered that the battery wasn't charging. So I wonder if there is an intermittent fault. I'm getting it checked out again!

does it have a lucas ACR alternator on it??

cos if it does the problem could be the center brush boring a hole through the slip ring