I ended up only with 23L in the FV at 1058 OG. I put 2L of still water in the FV to top up. I did this about 2 hours after pitching the yeast; a bit late now but hope I havent oxidised it with pouring the water in. Hopefully fermentation wouldnt have taken hold and it'll be OK.
First time I've used a bubble breather too. I'm only using it because the FV is one of those 4 inch screw top jobbies with a bung in it. What a racket its making, sounds like an aquarium bubbling away behind me.