Burden of Proof and Insanity Defence

I don't understand the circumstances in which the prosecution would raise the defence of insanity. Even if they did, bizarrely, do so, why would that change the burden of proof?

Hey! I'm doing some criminal evidence work and I would like something cleared up, it may seem obvious but its really confusing me haha.

If the prosecution was to raise the defense of insanity on the defendant, would the prosecution have the legal burden of proving it (by beyond all reasonable doubt), or would there still be a reverse legal burden for defendant to prove it (on a balance of probabilities) even though the prosecution raised it?

Not sure if i worded that well. I hope you can help, thank you!

In limited circumstances the prosecution can raise the insanity defence, for instance when the defendant raises diminished responsibility as a defence. In this situation, the legal burden rests on the prosecution to prove to a standard of beyond reasonable doubt.

Still don't think I've understood. If the defence raises the partial defence of diminished responsibility, why would the prosecution wish to raise insanity (a complete defence)? I cannot see anything in Blackstone about this or anything that suggests the burden of proof would be different even if it did, exceptionally, happen.

The only case my limited researches have turned up where this happened was a case from Australia, R v Ayoub, which seems to turn on entirely different statutes and rules governing defences than we have in England & Wales (although it's difficult to tell because the decision isn't on Austlii or Westlaw, or at least I can't find it on my phone).

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