it would really depend on the battery you have installed

u have to match the charger to the amp hour capacity of the battery (assuming the battery is the correct one for the machine)
thats about it
im not sure if its the same in the uk but here there is stampings on the side of the batt that you can figure out its capacity

i say call your local battery dealer and have him match a charger for you

here is some info from the webpage gbbattery com/FAQ

*these are just examples and may not apply to the machine mentioned above

FORMULAS:

Charger Model# Breakdown: Example, Model GB100-3-18-750 = GB brand, 100% rated, 3 phase, 18 cell (36 volt), 750AH (Ampere Hour).

Battery Model# Breakdown: Example, Model 18-125-13 = 18 cells (x2 = 36 volt), 125AH per plate, 13 negative plates.

Calculating AH Capacity: Negative plates per cell - 1, / 2 x AH per plate = AH (Ampere Hour).

Example, 18-125-13: 13 - 1 = 12, / 2 = 6 x 125 = 750AH

Calculating KWH Capacity: AH x Volts (2v per cell) = WH (Watt Hour) / 1000 x.97 (efficiency factor) = KWH (Kilo Watt Hour)

Example, battery model 18-125-13, 36V / 750AH: 750 x 36 = 27,000 / 1000 = 27 x.97 = 26.19KWH.

(13 - 1 = 12 positive plates per cell)