When was the last time you stripped and cleaned the carbs? How old was the fuel pump? It could have been the case that this work needed doing and the lay-up just accelerated that work. Running the car up to temperature once a month will help, but we appreciate that's not always possible. In which case, you'll need to treat each car as if it has been laid up for six months. That means removing the spark plugs and lubricating the bores before attempting to turn the engine over by hand. This helps with any stuck pistons. Once the engine is free and moving, clean, gap and replace the plugs (make a not of which plug belongs to which cylinder if you're worried you might forget). Then remove the air cleaner and use some Easy Start (or similar). You'll want to check the carbs are free and working. You could also use a battery jump pack (or jump it from another car) to make sure enough power is available and prevent your batteries from being drained. Do the cars have inline fuel filters? If so, these may need cleaning/fitting before being put away. Finally, you could try a fuel additive to stop the petrol from going off or the ethanol separating.