It rather depends which sort of lead acid batteries you are charging.
There are two basic types; flooded and SLA.
Flooded will put up with crude charging schemes, but you need a current limit and you can't let the charging process continue until electrolysis reduces the electrolyte level to below the level of the plates so they dry out.
SLA are different. They put up with a certain amount of overcharging because they can recombine the oxygen and hydrogen produced, to a certain extent. They can't tolerate much overcharging or they dehydrate or start to leak. Then there are a couple of sorts, those intended to sit there with their float voltage applied and rarely be called to discharge, as in burglar alarms, and those intended for cyclic use, as in halogen lanterns.
There are two basic (sensible) schemes for charging them. Bear in mind that the maker's data sheet should be consulted for the float voltage and the cyclic charging voltage. There's also a gassing voltage which you ought to avoid with SLAs. All of these voltages are slightly temperature dependent, and vary slightly from maker to maker, although there is a latitude. Burglar alarms I've seen don't
Fixed voltage, current limited. The voltage is set to the float voltage and the current is limited to below what the maker recommends. The voltage is something like 2.25 to 2.30 per cell @ 25C and increasing by 4mV/cell/degree C as you get colder. Unless you have to deal with fluctuations in temperature of that sort, you can get away with an accurate fixed voltage current limited charger set to , 2.3V per cell based on an LM317 or an L200 voltage regulator chip. Ideally, the float voltage would be temperature compensated to track the ideal float voltage, which really rules out a wall wart type charger.
Two level chargers. There are chips such as the UC3906 which you can set up with resistors to implement a charging regime. They monitor the battery voltage and do an exploratory charge if it's below a certain level, that's to avoid catastrophes from charging batteries with dead cells. They then charge at a bulk rate, which you determine with a resistor, they switch to a controlled overcharge mode, apparently necessary to reclaim all the capacity of the battery with the voltage set at 7.3 to 7.4V for a 6V battery, then when the current drops to 1/10th of the bulk rate (50mA if the bulk rate is 500mA), they drop the voltage to the float level. They are temperature compensated. I'm not sure if they are designed to work with 2V SLAs. Cyclon SLAs are a bit different and have a slightly different recommended voltages.
Burglar alarms I've seen have a factory set float voltage and don't include temperature compensation, but the batteries seem to survive well enough. The chargers chucked in with the x million candle power halogen lanterns are truly awful, along the lines of a 9V unregulated supply, about 14V O/C, with a resistor in the lamp to limit the current. It's no wonder that there are so many reviews complaining that they are useless and fail after a month or two
The other thing about lead acid batteries is that they degrade if less than fully charged and so need to be left on an accurately set float charger, or regularly topped up, and they suffer with deep discharge. These conditions don't fit with the pattern of use most people expect with a rechargeable torch, or motorised wheel barrow, much less with the chargers they chuck in with them.
There's a world of information on charging lead acid batteries on the WWW.
I believe Maplin sell a multi voltage lead acid charger for SLAs, but I wouldn't trust it myself.