I have my car lowered 40 mm all round with 15" alloys. Handling has improved. I will be getting a strut brace shortly as this will improve the handling even more.

Corsa is notorious for its crap handling with factory setup.You'll need to use some new springs to lower it 3-4cm and use stiffer suspension.Don't overdo it though because you'll end up with more problems than you have now!Also buy some decent wheels,I wouldnt use anything bigger than 15" because performance will be down.Also use wide tyres,at least 185.And antiroll bars both front and rear (very important).I'll give you my setup for example.yellow apex springs,yellow bilstein suspension,14" alloys with 195 tyre.

My corsa B handles absolutely crap as the speed gets up!

There are some country lanes near me with some fairly long straight sections.... you can reach 80 if your a naughty boy...... :P

I drive at exactly 60mph :P and because the road is a bit lumpy the car goes all over the place!!! You constantly have to fight with the wheel!!

What would help this affect? Stiffer suspension i'n guessing and possibly alloys?