Humax/Sony TV. Recording start alters TV input. How can I stop this?

I have Humax FoxT2 connected to Sony KD-40W5810.When the Humax is set to record, some 10/15 mins before the start of recording the input on the TV is changes to the Humax. As a result whilst watching the TV via the TV tuner the picture is lost.How do I stop this happening?

The TV aerial input is split at source. I have tried both HDMI and Scart connections but the auto input switching still occurs