i had the same problem today,as i was in the middle of a job my vikan brush became quite frozen,"it was realy bl**dy cold in gloucester"when it gets like that i give up,its too cold ,end of.
i also normaly take my poles,hose etc keep them in the garage over night if i think its going to freeze,forgot to do that last night,poles hose etc were frozen solid this morning,so another v-late start,and with brush freezing another early finish
i know a lot of guys leave a green house heater in their van over night,but im a bit too worried about the chance of a fire to do that,i used to use very small electric heater,but my movano quite big,would cost a fortune trying to keep that warm.
ps hope your dad is ok