Unfortunately I don't know what the plant actually is but I've put a
couple
of pictures here.

It is a schefflera.

Any ideas what might have caused this, (possibly central heating ?) and
how
I can get it to bush out again at the bottom. I thought either

Yes, undoubtably the heating perhaps you could of prevented it by supplying
a little more humidity.
Get a large pot saucer, bigger than the pot's top diameter. Fill it with
gravel then keep it topped up to HALF the depth of the saucer with water, so
the pot never actually stands in water. It will tend to sink into the gravel
a bit.

how I can get it to bush out again at the bottom. I thought either
1. Pinching off any new growth at the top
2. Cutting it back to about half it's height and pinching off new growth
3. Cutting off the smaller side shoots that have only sprouted this year
(2002)

You can propagate cuttings in the summer if you use a heated propagator
(i.e. with bottom heat.).
You could air layer any of the mainstems near the top of the bare bit. You
do this by getting a piece of plastic, wind it round the stem loosely then
tie at the bottom, pack into the plastic a handful or two of damp spagnum
moss around the stem and tie the top of the plastic around the stem.
Overtime you should see condensation inside the plastic, that's great that's
how you want it - damp and warm. Make sure the spagnum never dries out
completely. You should fairly soon see roots forming in the bag, it might be
wise to cover it with paper or several sheets of black dustbin liner to keep
the roots dark- it's important it never dries out completely !!! make sure
there are loads of roots in the bag, , then put your hand on your heart and
chop the main stem off below the bag. Pot it up and Roberts your Dad's
brother ! Don't be tempted to treat it special, keep it in exactly the same
conditions as when it was growing roots i.e. don't move it anywhere else -
you want to minimise environment change otherwise it will probably just
start shedding leaves again and you've wasted your time.

Simon