I harvested some custom Saison yeast from a particular brewery I like. I made a starter and have kept in the fridge. As there was only a small amount in the bottle (as expected), the amount of yeast is quite small - maybe 3mm layer on the bottom of an average jar. To increase my yeast sample for a future brew, do I just tip the liquid out, re-make another starter, adding this yeast? Can I do this a few times until I have a good amount?
Do a step starter.
Start around 500ml and double for each step until you get your pitch amount needed.
Yeast from bottles always has to be stepped up several times. Most of it will be stone dead (having been through the wars in both the fermenter and the bottle and having aged for far too long) so you'll need to give those few remaining live cells every chance they can get.
However, keep in mind that many breweries use dedicated bottling yeasts these days. In other words, the yeast in the bottle is NOT the yeast used in the fermenter! Bottling yeasts tend to have a fairly neutral flavour profile and not ferment maltotriose to an appreciable degree (thus respecting the beer's body and flavour profile) but are completely unsuitable to brew a beer with.
Whether or not this is the case with your yeast deposit remains to be seen; some breweries will tell you while others will not. In this case trial and error (or, as I can state from personal experimentation along these lines, trial and terror) is the only way to find out if the yeast in your bottle is the real deal or a bottling yeast.