There was also one window used as a cat flap I believe. The whole family were sitting ducks as Jeremy must have realized. The intolerable dreariness of his weekday existence,mitigated only by intermittent spliffs of cannabis pushed him over the edge as he realized a way out of his predicament,where fear of failing Nevill was the real reason,concealed by the pretence that he was putting them all out of their own misery. Jeremy yearned for a fresh start,which meant destroying everything in his wake. If he could have burned Nevill and June's bodies on the pyre with the bedding and the carpets he would have..
In court a will of Nevill's was shown.
It basically said Jeremy would get his inheritance if he had earnt it. That could mean another 20 years of 14 hour days. Even then he would have to share the will with Sheila & the twins, who had nothing on the farm.
As mentioned June was also thinking about changing her will to give the twins more.