Surreptitious recording is only useful in any subsequent argument. In particular, people who are retracting something they have told you - perhaps after being pressurised by the police - will say that you did not introduce yourself properly, that you lied and that you tricked them. The surreptitious recording will be evidence to counteract such claims. It will also demonstrate to people that you acted fairly and that the witness gave the evidence freely. Moreover, it demonstrates that the witness is prepared to lie - and that is at least something in your favour.
If you make an agreed recording - either audio or video, it is best to first transcribe it, then take it to the solicitor. Ask the solicitor to draw it up as a sworn statement - and then persuade the solicitor to take it to the witness for it to be sworn to and signed. This is your best method of working. Only Commissioners of Oaths can actually finalise a sworn statement - but your solicitor may be one, or will know one.