"There is no UK law stating the age at which a child can be left at home alone. However, parents can be prosecuted for wilful neglect if they leave a child alone or unsupervised "in a manner likely to cause unnecessary suffering or injury to health" (Children and Young Persons Act 1933)."

"Babies should never be left alone, even for a short time. If you notice that a baby or a child under the age of nine has been left on their own, contact the police on 999. They will go to the house to make sure that the child is safe from harm."

"Before leaving an older child alone, parents must take into account the child's age and maturity, their ability to cope in an emergency and how they feel about being left alone. Most children under the age of 13 are not mature enough to cope in an emergency and should not be left alone for more than a short time."

From the NSPCC help and advice page

Many parents who lack support from others or who are in difficulties of some kind (the financial necessity to work etc) may sometimes have to make difficult decisions. In my experience I don't think such circumstances would ever be deemed to be "wilful" neglect and any "authority" involved would attempt to see that the parent received appropiate help.

I still maintain however that leaving three children under the age of 4 alone in a strange environment (and refusing the baby sitting service readily available) to go out each evening drinking with friends is unacceptable behaviour; not the least as these "professional" parents would be well aware of the guidelines.