I have been called to a 'lack of capability' hearing. Will I be able to take someone with me?

The nature of the hearing is crucial as to your legal right to be accompanied. If this hearing is to be an informal chat between you and your manager with no likelihood of any formally recorded warning being an outcome, then you would not have a legal right to be accompanied.

However, if this is a call to a formal meeting, either as part of the disciplinary procedure or as part of a 'work capability procedure', that assumes the form of a disciplinary hearing giving rise to the potential of formal disciplinary action being taken, then you should have the legal right to be accompanied by a trade union official or a worker of your choice.

If this is a call to a hearing as part of a formal company procedure it may be that the procedure itself has a provision for you to be accompanied, which may reflect your legal right or provide additional options.