Hi Simon,
The lead acid cells have a potential of 2.1 Volts - multiply this by the six cells in a car or leisure battery - and the no load voltage should be 12.6 Volts.
This voltage is modified by the different chemistry used by different manufacturers, but will always be in the range 12.6 to 12.8 Volts.
However for 24 hours after freshly charging a battery, the voltage will drop from an unrepresentative high of around 13.6 Volts to around 12.6 volts as the cells settle down.
Then each day the battery will lose around 0.5 AH of capacity, so your 85 AH battery will be half charged after about 7 weeks - and then the battery should be showing close to 12 volts...
Recharge it as Eddie says - once a month is good practice - because the battery will "self harm" if allowed to sink below 50%
Robert
A freshly charged battery