Car naffing me off

I apologise if this message causes irritation but I really have trawled for days through the posts and can't find anything relating to my problem.
I'd post a picture of the exploded view showing where I have the problem but can't work out how to get it into here.
My 2001 td5 has a diesel leak. Its above the fuel filter housing, a screwed joint between the head and the fuel cooler pipe. I have replaced the copper washer but the drip remains and is on the head side of the washer. I can't tighten it any further either. It's been suggested, by a garage, to put some gasket sealer on it but thought I'd see what the experts say first. All help very much appreciated.

Got a TD5 Discovery - last year wouldn't start during snowy season and had the glow plugs replaced and all was great. This year its snowy again and yes it won't start, this time it stops itself even trying and all the lights on the dashboard go on and off. Has only been sat for two days after a long run so any help much appreciated before I beat the crap out of it. Thanks