John Greenwood wrote:

1. Is there any reason why I cannot cut the hole for my sink in-situ, i.e.
with the worktop in place and screwed to the carcasses. This seems easier to
me, avoids having to move the worktop and risking it breaking due to it
being weakened by the cut out.

Yes, there is a reason - you'll most likely be cutting it with a
jigsaw, so you need to cut from below in order to avoid tearing
the lamintate off.

You could use a downcut blade, but IMO they are difficult to
use, and don't produce great results (because they're constantly
pushing the jigsaw away from the surface).

The worktop won't break unless you're very careless, they're
pretty strong.


2. Should I seal all exposed chipboard? Is so with what varnish or PAVE?

I use PVA, others use silicone. Both will work fine.


3. Once in place should I seal the edge with a silicone sealant before or
after I tile. I think maybe before but then what do I do after it has been
tiled? Seal it again?

Is the sink sitting on top of the tiles?

--
Grunff