Hi @Doog, as an HR Director, let me give you some advice.
As said above, Capability and Conduct are different processes - conduct is typically dealt with under disciplinary processes (when you "won't do" what is required), Capability processes apply when you "can't do" (i.e. training gap or some other reason eg disibility). The investigation they undertake is likely to identify whether there is a capability issue or you simply choose to disregard the requirements of your role.
Capability procedures, otherwise known as Performance Improvement Procedures, should be used by good employers to help employees reach the standard required - usually by putting a Performance Improvement Plan in place with clear objectives, training, coaching and support specified - followed up with regular check in reviews and ongoing feedback. Typically I will ensure my managers put these in place for 12 weeks (with 3 x 4 weekly reviews). If performance has not improved after 4 weeks, we issue a written warning. If performance is still insufficient at 8 weeks, we issue a Final Written Warning. At the 12 week review, we would dismiss if performance is still below expectations. I do expect my managers to document concerns informally approximately 4 weeks before entering into formal processes to give my employees the chance to improve without entering formal processes (something I'm very passionate about, although not legally required - I hate managers coming to me to talk about "getting rid" of an under-performer when they have zero documented evidence of coaching for improvement of giving feedback to the employee). Employment lawyers tell me that tribunals would not look favourably on performance related dismissals if employers can't demonstrate 4-6 months worth of performance management, but I rarely let processes run that long and make a commercial decision to usually terminate employment at the 12 week mark (unless there is a protected characteristic).
So, looking at your case, there is clearly an under-performance issue. As long as you don't state in investigation that you know what you should be doing, but just choose not to do it, then this will be dealt with as a Capability / Performance Improvement matter - not conduct. I would recommend you say at this stage that you are disappointed that you've been taken down this formal route at this stage without any informal coaching or support from your line manager or QA to improve - that should push the buttons of your Head of HR.
Now here's the more dirty stuff?
Do you have 2 years service? If not, and your employer has an agenda, they can fire you and you don't have any comeback at tribunal unless you have a protective characteristic and are claiming discrimination (are you gay, an ethnic minority, pregnant etc).
If you are in your probationary period, they may decide just to terminate your employment without following any Performance Improvement Procedure - I probably would, so be wary
Employers will always tell employees being put on a Capability / Performance Improvement Procedure that this is being done to "help the employee achieve the performance levels required" - your employer might genuinely mean this, but (trade secret) there are 2 types of performance improvement procedures - those designed to get someone out the company and those designed to genuinely support someone to improve. You need to figure out which it is - based on how achievable the targets are they set and the level of support offered (remember that if performance doesn't improve to the required level you will be dismissed). If its a genuine process and you are confident you can achieve expectations, then you have no problem. If on the other hand you get the sense that you are being managed out with unfeasible expectations, I'd begin looking for a new job - they will take several months to get you out (during which you can be looking for a new role). Whilst you could consider a tribunal claim post dismissal, there are no guarantees of success and any payout would be significantly reduced if you found another job in the meantime.
I do hope this helps. Let me know if I can be of any more assistance.