snipped
Perhaps you would give me your opinion on another matter on which I've
been posting to a DIY ng.
The erection instructions for my greenhouse say it should be anchored to
the ground by drilling through the steel flange on the base into the
concrete slabs on which it stands.

What size paving slabs? If you drill into small ones you might loose one or
two to cracking. To give yourself the best chance of not cracking them, use
a new or sharpened masonary drill bit, (diamond tipped if you've got one),
set your drill to hammer and try not to take too long drilling the hole, or
keep stopping to blow out the dust, as this blunts the bit pretty quickly.

I can see that I will have trouble in getting the screws in as well as
drilling the holes. I've been provided with 10 x 2" wood screws & rawl
plugs to be placed three each side of the 8' x 6' base and two each end.

That sounds about right, but make sure you use brass screws, as it's going
to be in such a damp area, steel screws will rust.

Since I can't budge the greenhouse with just the 3mm toughened glass in
the roof. I can't imagine the wind shifting it with the side glass
fitted as well. I'm wondering if there isn't a heck of a lot of overkill
here and how many gardeners do anchor their greenhouse down. My garden
is quite well sheltered by hedges and is on the leeward side of the
house..

Although it sounds like you've got the greenhouse in a pretty sheltered
position, you still have a large surface area for the wind to push against,
think of the sails of a yacht and you've got a good idea of how your
greenhouse could behave in a strong wind.

Having said all of that, I think most metal greenhouses (those that aren't
on a brick base) are secured using ground anchors, which are set into
concrete and bolted to each corner of the greenhouse.

HTH

Cheers

Nick
http://www.ukgardening.co.uk