my TW makes an occasional clatter on startup, which others here think is the timing belt tensioner, and I beleive them. Doesn't do it often so I'm just keeing an eye out for it. The Q is: are there any oils or additives that have a history of fixing this problem?

Nope. The belt tensioner is outside the engine (but under the plastic covers) and no engine oil gets there.
If it clatters, turn the engine off and start again. If it's quiet, it's definitely the tensioner.(That's Mal's advice.)

If it is the tensioner, get it looked at as soon as possible, the clatter you hear is metal hammering on metal and if something breaks, the cam belt may jump teeth and damage the engine. And I don't want to think about the damage if the broken bit gets caught in the moving belt and does more damage.

mine needs a new one too, what sort of price am i looking at for a genuine unit? the local dealership near me seems to be a pretty pricy place

$400 would be a reasonable price supplied and installed genuine at a dealership.

oh ill install, how much just for the part?

$195 inc post for genuine one

Nope. The belt tensioner is outside the engine (but under the plastic covers) and no engine oil gets there.
If it clatters, turn the engine off and start again. If it's quiet, it's definitely the tensioner.(That's Mal's advice.)

If it is the tensioner, get it looked at as soon as possible, the clatter you hear is metal hammering on metal and if something breaks, the cam belt may jump teeth and damage the engine. And I don't want to think about the damage if the broken bit gets caught in the moving belt and does more damage.

OK I guess I just better bit the bullet and get it done.