I don't know if sycamore syrup is different from maple syrup, but maple syrup can last for months refrigerated, sometimes years. Mold won't grow in it because of the high sugar concentration, but it can grow on top if exposed to air. I have found (and read elsewhere) that it can just be skimmed and used without problems if it develops a bit of mold.
A boiling water bath and pre-sterilized jars would make mold much less likely until opening, if you don't want to store it in the fridge. I'm not sure I would want to put the syrup in a pressure canner, as I don't know how much it might boil and foam while out of sight. I wouldn't want to risk clogging the vent.
Glenn is right. As long as the sugar content is high enough, you won't have any trouble with mold or bacteria. Just keep it airtight in a cool dark place and it will be just fine. If mold appears on the surface of the syrup, just skim it off. If you feel weird about using it if it's molded a little, just reboil it and it should be good to go. I buy maple syrup from a place down the road from me by the gallon. One gallon will last me a couple of years, but it's the most economic way of buying it. I sometimes divide it up into multiple jars and store it in the cabinet.
One thing about mold: White mold is usually fine as long as it's not super fuzzy looking. Green is not great, but likely harmless. Black mold is no good. If my syrup had black mold, I'd probably not use it, just to be on the safe side.
Do you have a way of measuring the sugar content of your syrup?
No Craig. We will only be making tiny amounts so we have no proper kit at all. If we can collect a respectable amount maybe i will invest in some decent gear. Having said that the tapping kits were beautifully made and work superbly. (Touch-Wood in the UK)
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Never made sycamore syrup, but have made birch syrup. The process is the same. I boil my sap down outside in order to not have all that water vapor in my house. When I get the syrup almost to the finished stage (the point where it would be easy to scorch) I bring it into the house and finish off on the stove top. I filter it through multiple layers of cheesecloth or an old clean white teeshirt directly into my jars while it is still very hot. I use mason jars and immediately put on the lids. As the syrup cools the jars seal. I've kept it on the shelf for several years with no ill affects.
I know with maple syrup it takes approximately 40 gallons of sap for a gallon of syrup and with birch it is approx 100 gallons of sap per gallon of syrup depending on the sugar content of the sap. How much sycamore sap does it take to make a gallon of syrup?
ETA: I see where yours was "watery". I would guess it wasn't cooked down quite enough. A water bath may be appropriate in this instance. Do some research on maple syrup production and how to tell when your sap is at the proper stage. Most maple syrup produces use a hydrometer. You can also use temperature of the syrup to tell when you are close as the boiling point rises to close to 220* if I remember correctly and there is also the spoon method where you want it to sheet off the spoon.