I intend fitting new core plugs to the short 3.5 motor I am rebuilding - I have removed the old ones already. I was going to drift them in with a hammer and socket - is the right way? Do I need to use any sealant? On another note, the rectangular plastic screw cover that hides the screws on the front of the instrument binnacle on series 2 cars, how do they clip on? Any thougts would be appreciated.

You should just be able to carefully tap the new core plugs into the
holes - after cleaning the holes to remove roughness and crud. No
sealant is used, at least I didn't use any. The core plugs I removed were
paper thin! I've still to do the two core plugs at the back of the block,
the gearbox and bellhousing/flywheel (eek!) need to come off to access
these

As for the screw covers. I've just looked at a spare series 2 dash I've
got in the loft. The covers just pull off - they are flat at the back with
a raised circular moulding which holds the cover in the depression
surrounding the screw.