Hesitating/missing 4.6

Hello all,

Travelling to work this morning after the car had been standing in heavy rain since Sunday, I was travelling at about 70mph when the car started to hesitate. On slowing down it seemed to be missing slightly and lumpy when idling in gear in traffic. On arriving at work it appeared that one plug lead was slightly loose. I pushed it on but the problem seems to be still there.

Any ideas?

thanks

Steve