How should steal seal head gasket repair be used, does it work?

My car - 2006 ford focus 1.8 tdci. It's had around £2000 of repairs in under 2 years of my ownership

Car had chemical test 15 months ago, indicating bad head gasket.
however 15 months on no symptoms of hg failure, no water/oil mixing, no power loss, no smoke, just have to top water up every week or so.

I would replace hg but car has other problems so don't consider getting it done though I'm constantly debating it.

anyway, this steal seal says pour in radiator but my car has no radiator cap, so would I just need to pour it into the coolant header tank? Is steel seal an y good?

Don't waste your money. You have spent enough already on the heap. Trade it in for a Citroen/Peugeot diesel. Road test the next car properly.

Here's a post on how to use Steel Seal with the most chance of working.
Since you don't have a radiator cap, drain some coolant, remove the upper rad hose and pour the Steel Seal in the rad that way. Don't pour it in the reserve tank.