Yes, electricity and heat are separate within the Square. This is fully explained in a post I made HERE.
So it is possible E.On want a reading for your heat, and Spark for your electricity. As you say, your electricity meter is in the hallway cupboard so it is easy enough to sort out Spark. For E.On though, this is strange as your heat meter is a smart meter than E.On can, and normally do, read remotely: They may need a phone call.
One last thing: Are you renting through Foxtons? If so, I may know what is going on. As a 'service' to their customers, Foxtons arrange for the electricity to be turned on for new renters with Spark energy (ordinarily you would have to arrange this yourself after the previous tenant cuts the juice when they move out). It could therefore be that E.On were supplying power before you moved in, and after Foxtons switched to Spark, both companies need a meter reading to complete the switch over.