Daimler V8 250 Core Plugs

[Taff_b]

I have had a cylinder head core plug 'fall' out whilst driving. I don't think it has caused any damage, but I need to fit a new one.
I have bought new plugs, but they are a bit loose.
Now I am experienced in changing the dish type plugs on ford and mini engines, but the ones on my Daimler V8 are just convex, and they are not an interference fit.

Am I right in thinking that they have to be hammered in so that they become flat, and that this gives an interference fit?

Do they need to have a sealant applied when fitting?

[Taff_b]

Thanks,

Can it be done in situ to the one fitted at the front of the head?