Hi Tony H (to distinguish you from the other Tony - you wouldn't want to be mistaken for him, now would you?).
Up until 2011 I was sailing my kit-built Storm Petrel (14ft), probably the tippiest Swallow Boat ever? I found water ballast a great help. I used semi-rigid (= can be sort-of squashed down flat when empty) 15 litre cuboidal camping water carriers, up to 4 of them, stowed either side of the dagger board case. I think you'd find 50 litre tanks too big/heavy: they'd have to be filled in situ, meaning access to a hose etc, and emptying them would be a big problem. Another advantage of multiple smaller units would be that you could move them around, at least a bit, to adjust trim if you wanted to.
I filled mine outside the boat after launching, using a garden watering can with about 2ft of hose on the spout, then lifted each into place - quite manageable. Incidentally, trying to dunk them under the water to fill them was useless - they just squashed flat and virtually no water went in! That's probably very basic physics, but muggins me had to find out the suck-it-and-see way.
Using six of them would give you 90kgs of trimmable ballast really low down in the hull, involve a modest outlay, and probably add about 10 minutes to your launching schedule. Water ballast really is great, because you don't have to take it with you and you leave it behind when you come home! And it always surprises me what heavy stuff water is.
Very best of luck. Can you let us know how you get on?
Michael