Hi there and welcome.
I notice that you say 200tdi, yet in your profile your defender is a 95 which if i'm right would make it a 300tdi????? It probably makes no difference but ya never know :-).
On the subject of radweld i'm not a lover of it personally, but that doesn't mean it wont work.
The timing belt cover if i remember rightly has a chnel in the back for the water to pass through, and a gasket is incorperated into the timing belt cover. Baringin mind that i'm refering to a disco 200tdi engine and i believe there is a slight difference.
if you take a look at
[ HOW TO ] 200tdi Timing Belt
On the 6th pic you can see the channel on the right hand side. I would suspect that this is the gasket thats leaking IMHO.. hope it helps..