I think paying online by bank transfer requires a code, given by HMRC.
You could just send them a cheque, with your details, and I suppose they would accept it. If you are happy to pay the Class 3 level of contribution, which appears to be the default option, I imagine you could do that.
But you should be certain of which class of contributions you are making, because the difference is quite large ( 165 a year, as opposed to 824 a year)
I think that requries an assessment, which requires an application using form CF83.