Jeßus wrote:
I have a large garlic patch this year (approx. 300), a friend reckons
I should leave them in the ground to regrow next year... and they
should be even better than this year.

If I do this, I may harvest every second or third bulb and leave the
rest as suggested above.

Is this a good idea? Thoughts/suggestions appreciated.

No.

First there is the risk of rotting in a prolonged wet spell while the heads
are dormant.

If they survive the heads will all sprout from each clove and this is a
waste and it will crowd the next generation and you will get less next year
not more.

D