From Hitchcock's site:
LU/54028423 BATTERY 6v (DRY) 121 x 61 x 131mm Redditch models after 1958 £19.50
I assume the unavailable one is prior to 58?
LU/PUZ7E BATTERY 6v DRY 120 x 90 x 160mm Blk Exide type Redditch mods
If that one is about the right size, 4.7" long, 3.5" wide and 5.1" high, a Yuasa 6n6-3b-1 should work. Its a 6 amp hour, 6 volt battery, its 3 7/8" Long, 2 1/4 Wide and 4 3/8" High