The Revenue don't use collection agents. They will take Court action to enforce payment or may seize goods. See this link:
http://www.hmrc.gov.uk/howtopay/dontpay.htm
Usually, though, enquiries are settled by means of an offer, which can be significantly less than the full amount owing. See here:
http://www.hmrc.gov.uk/manuals/emmanual/EM6214 .htm
The revenue don't mess about. You haven't paid your proper wack and they want from you what everyone else has paid, and more. But if you really can't pay they will listen to your reasons. You might be able to get away with teh penalty, at least.