Nationwide mortgage for self employed contractor

My husband is a self employed gas fitter. He contracts to a company who pay him weekly and pay his taxes for him. We have applied to the nationwide for a mortgage which is 30% Ltv. We have been offered our mortgage in principal but now the nationwide are chasing everything but the sun. The company he contracts to has provided a employment reference saying he is paid £x weekly And he is self employed. Nationwide are now requesting an accountants certificate ? Where do we get one of these from - I do his online self assessment which is pretty straight forward and as this company pay his taxes we do not have an accountant.

Hope someone can help and sorry if this has been posted before I have searched for help.

Many thanks