The IHT nil rate band is now �285,000, so if the house is valued at �400,000 the bill (at 40%) is �46,000. The nil rate band applies to the value of the deceased's estate as a whole. So if your father left other assets as well as the house his total estate would be more than �400,000 and the inheritance tax more than �46,000.
The tax has to be paid out of the estate and the executors are responsible for seeing this is done. I don't know of any way you can reduce the liability now, but you could discuss it with the solicitor. In some cases the Tax people accept payment over a period of time, but interest is added.