How many people are living in the flat?
To calculate overcrowding, children under 1 are not counted and children under 10 count as half.
Your living room is counted, as well as the bedrooms, and large kitchens.
Assuming you have one living, two bedrooms and a small kitchen, your flat is 3 roomed and can accommodate 5 people.
If your 5 children are between 1 and 10, they count as 2.5 people, with yourself that is 3.5 and if you have a partner, 4.5 so there is no overcrowding.
But if your children are older than 10 there is still not statutory overcrowding as this is considered to be natural growth.
As you are working and self supporting, there is nothing to stop you finding alternative and better accommodation for yourself and your family.
http://england.shelter.org.uk/get_advice/repai rs_and_bad_conditions/overcrowding
That link sets out the rules.