Caterpillars on my broccoli - help!

I obviously didn't spot the eggs - but found the leaves swarming with caterpillars munching away. I've removed the leaves - still with the caterpillars on them (don't want to kill butterflies - I like butterflies) and put them in the woods behind my house - but is there a method of killing the rest of the eggs before they hatch?
I've sprayed everything with washing-up liquid in the meantime on what I think are the eggs, but to be honest I'm not even sure what caterpillar eggs look like!

Thanks, Sunshineband - I'll see if I can find their eggs.
Should I have covered the bed with netting to prevent this?

A few ideas in these threads
http://chat.allotment-garden.org/index.php?topic=34665.msg415429#msg415429
http://chat.allotment-garden.org/index.php?topic=59123.msg691492#msg691492

the only trouble with netting now is that any it is harder to keep an eye on and remove any eggs and caterpillars.

for the rest of the year it may be better to leave un netted and keep checking and picking of the pests.

if you net next year make sure the netting is tight enough and not gaps so they don't get through, they seem to be able to get through gaps a tenth their size.

Mealy aphid?
https://www.rhs.org.uk/advice/profile?PID=656

If they fly up in a cloud they are cabbage whitefly
https://www.rhs.org.uk/advice/profile?pid=482