Low voltage cut out switching for a 6v input SLA to 5v output circuitry - or alternative ideas
It seems I may have found a solution ( not fully tested as yet but looks promising). 

I used a Mosfet based low battery cut off controller and "resistor-ed" it to suit a 6v SLA battery. I researched a Jaycar Electronics Kit ; KC-5523 (called Battery Saver) and combined with a bi color LED indicator module kit (also Jaycar) I have a fully functional unit which switches off @ selectable discharged state. (6v)

Components are: a charged 6V SLA , powering the 7" monitor / door phone, via a 5v LM2940 LDO regulator (.47 cap Vin side and 22 cap on the Vout). The Mosfet controller, the battery indication LED circuit, a 600ma 6v charger (which when plugged in disconnects ground to Monitor - shutting off power) As the battery is in low charge @ around 6v, it still functions on standby, but if the doorphone is used in normal power mode @ around 1A, the current exceeds the battery discharge rate, quickly reducing voltage and activating the mosfet switch, cutting power to the unit and saving an overheat situation with possible fried electronics ( I did install a big heat sink on the LM2940 just in case , and vented the case) 

I havn't tested how the charger reacts as yet, hopefully it will function normally and when charged, will reset the mosfet switch ( requires around .5V above cutoff approx) I don't understand the Hysterics, but @ .5v I am assuming cutoff history is the point of reset. ... hopefully the project will be complete with some tiding up. thanks to those that had thoughts on the project.