Courgette seed saving

I’ve a question. How big do you let your courgette get for seed saving?
I’ve done the isolation thing, and I’ve used the male flower as a paint brush. Do I leave it to get a little bigger than normal. Or do I pick at normal size and in either case store it and allow the seeds to ripen.

I thought you left them on the plant until they went yellow and completely ripe - which stops the plant producing more fruit as it thinks its job is done.

Real seeds advice for cucumbers..........

Cucumbers need to be ripened well beyond the edible stage.  They will become much fatter, and green varieties will turn a dark yellow brownish colour, white varieties a paler yellow. Keep for a week or so after picking to let the seeds mature fully.  Then cut open, scoop out the seeds and surrounding pulp into a jamjar, add a little water and stir well.  Leave the jar on a sunny windowsill for 2-3 days for the seeds to ferment.    On the third day, fill the jar fully with water, and stir well again.  The good seeds should sink to the bottom of the jar, leaving pulp, debris and empty seeds floating on top.  Gently pour off the water and debris, refill the jar, and repeat.    After a couple of rinses, you should be left with good seeds at the bottom of a jar in clean water.  Drain off the water, and spread out on a plate to dry well.

Both melon and cucumber seeds will last for several years if dried well and stored somewhere cool.

EDIT This is from RS's leaflet

"Leave on the bush until fully mature at the end of the season. Rinse the seeds off and spread on newspaper to dry".

I’m using real seeds advice. pumpkin, courgette, marrow & squash seed saving is all in one and they use a pumpkin as the example.  This is what it says 

Beware that pumpkins, squashes, marrows & courgettes will all cross readily with each other.  The best (usually only) way to save pure seed on a home scale is to hand pollinate one or more fruits.  This is very easy & will avoid disappointments with lumpen squash/courgette crosses.  The explanation given here is for pumpkins, but applies equally to squashes, courgettes & marrows.

Pumpkin plants have two different types of flower, male and female.  The female flowers are the ones that will grow into pumpkins. They can be identified by the small immature fruit which should be obvious beneath the flower.  Male flowers just have a straight stem.  You need to transfer pollen from a male flower into a female flower, making sure that no pollen gets introduced from plants of a different variety.

One evening, when the plants are just beginning to produce flowers, find some male and female flowers that are going to open the next day. Buds that are just ready to open are much fatter than the others, and they have turned from green to yellow.

You need to stop these flowers opening, so that insects can't get into them.  The easiest way to do this is to gently slip a thin rubber band over the end of the petals, to hold them shut. 

The next morning  go back to the plants.    Pick a male flower, take off its rubber band, and tear off the petals.  Gently take the rubber band off of one of your female flowers.  Using the male flower like a brush, rub the pollen on to each section of the stigma in the centre of the female flower.

Then carefully rubber band the female flower shut again so that no insects can get in with more, 'foreign', pollen.  Tie a piece of wool loosely around the stem of the female flower, so that at harvest time, you know which pumpkins you have hand pollinated. 

Now leave the pumpkins to develop and ripen. After you have harvested them, keep them in a cool  dry place for another month or so to ripen further indoors.

Then cut the pumpkin in half, and scoop out the seeds, leaving the rest of the fruit for cooking as normal.  Wash the seed in a colander, rubbing it between your hands to get rid of the fibres, and then shake off as much water as possible.

Spread the seed out on a plate to dry.  It needs to dry as quickly as possible, but without getting too hot, for example on a sunny windowsill. To test whether the seeds are dry enough, try bending one in half.  If it is dry, it will snap rather than bending.

I think you'll need to leave the courgette to fully mature which will stop it making more courgettes.
I would personally pick this one and eat it and use it as practice for hand pollinating. Then towards the middle of the end of the season I'd hand pollinate another one and let the plant fully grow that one on to maturity. Then harvest and store it for a while (you'll effectively have a marrow), after a week or so open up and scoop out seeds and follow the advice above for pumpkins.

The problem will be, if you leave it too late, there won't be enough time for it to ripen fully and you won't get any viable seed. If you have a few plants, it shouldn't matter that you don't get too many from the one you are saving seed from. It needs to grow into a marrow and the skin get hard like a squash. You may need a saw to get into it when it is fully ripe.