I have installed the sink in a cutout in a laminate worktop (and
sealed all the cut edges with varnish). The sink has a foam type
gasket (self adhesive, came on a roll) running all the way round
that it sits on and has some really bad screw fixings that secure it
to the worktop.

Yes, all the sinks I've fitted seem to come with those fixings.


The question is
whether I should be sealing round the sink with silicon too?

Definitely.


The
installation instructions dont mention it, perhaps indicating the
gasket is sufficient to stop any splashes

It isn't.


(which seem more numerous than the previous
sink being shallower) going under - but I feel it would be better
with a bit of sealant round it?

Yup.


(It is a stainless steel sink in a black worktop -
translucent sealant?)

Yes, clear/translucent silicone is good.

The sealant doesn't need to show - it should be *under* the gasket, with
any
surplus wiped off once the sink is secured in place.

When I fitted mine, I had a job to get it to go down tight onto the
worktop - so used some strips of wood over the sink and G-clamps onto the
worktop until the fixings were in place.
--
Cheers,
Set Square

Dont you want to stop water reaching the gasket though (it isnt rubber,
looks like foam)?