You can't have a lump sum in lieu of maintenance, I'm afraid: you will just have to hope csa catch up with him eventually. I would take your solicitor's advice and try and get half the equity. You are entitled to the child maintenance anyway.
In the past a lump sum payment for CM wasn't possible because if benefits were claimed the CSA had to become involved. However, as I understand, because this requirement is being done away with in May 2008 now it should be possible for a couple to agree on a lump sum with the proviso that should CM is claimed in the future a sum of money is paid back.