Lidl have SLA chargers every now and then, I have one for the 18Ah Odessey battery on my car - BATTERY CHARGER ULG 3.8 A1

It can do 6v, 14.5v or 14.7v at 4 Amps, only about £14 I think. They have em in every now and then, but failing that do like said above, bang a power supply on and set it to 14.5v.