well this morning i started the van up and fired up the boiler and got to my first job and the batteries were reading 12.4 so i left my engine running for 30 mins(with the boiler running) then turned off the engine and carried on working for another 30 mins.checked the battery and it was reading 12.7!(fully charged).
all day ive checked after working with the engine off for an hour or so and the batteries havent dropped lower than 12.5.
so the batteries must charge up rapidly even when the boiler is running (if the engine is running).
im trying to see if i can go a week without having to bench charge the batteries.
this is all new to me ive never had a split charge relay fitted before. 
whats the lowest you can go without damaging the batteries?is it 12.2?(50% discharged?)
50% is generally the maximum discharge for a lead acid battery. This will probably occur down below 11.0v. Below 11v voltage will fall off quickly as capacity drops. The LL controller will warn you once voltage reaches 11v and stop the pumps at 10.5v or less.
Keeping the batteries above 12v is a good thing.
Bench charging a battery will give a deeper charge and maintain the lead cell in better condition as it returns more sulphate to a liquid state. You may well find that a bench charge once a week is enough.