Get a new genuine rover tappet cover gasket, they are considerably thicker
than aftermarket jobbies.  You have new rubber grommets, good.
Clean all the mating surfaces, get rid of any traces of old gasket that may
be baked onto the head/cover.  Before you fit the new cork gasket, smear it
with NEW engine oil.  Then when you need to remove cover to reset tappet
clearances it wont stick in places and disintegrate.
Some tappet covers have tabs at the front and back on the inside to stop the
cork gasket bending inward.  I normally put the cork gasket on the head
(hopefully stuck in place a little by the oil) then lower the cover over the
gasket.  Check the gaket is an even distance all round then tighten put the
top grommets in, then the washers that cover them and then the nuts.
The nuts dont have to be done up VERY tight, just tight or you will compress
them and they go harder quicker.
Start her up and check for leaks!
Then go for a blat!

chris

First, are the breathers clear on the engine. It's possible that there
is too much pressure forcing oil out of the rocker cover. Are you also
losing oil from the dipstick hole?

Another possibility, where is the vent on the rocker cover connected?
I've fixed two cars with the same problem by correcting the rocker
cover venting. One car had the hose from the rocker cover connected to
a plugged connection on the air filter housing. Hooking it up to the
carb where it belonged fixed the oil leak and some minor blue smoke in
the exhaust.

On the other car someone had drilled out the hole in the bottom of the
oil filler cap on the Metro style rocker cover. This appears to have
killed the slight vacuum that should exist in the rocker cover. A new
filler cap fixed it.

Cheers,

Kelley