Hi Big Steve
I use 3 things to align the dish, simple sat compass cost £5.00 including postage, sat signal finder and set top box itself. I moor up, fix the dish to my magnetic mount and roughly place it on the roof. Get out my trusty compass and stand well back for the boat (magnetic interference) then point the dish in the rough direction making sure that there are no trees or building in the way. Then using the sat finder signal meter I position the dish more accurately. Then using the satellite set top box signal strength meter the better half shouts so I can fine tune the dish direction. This usually takes about 3 mins at the most 5. Another tip use the sun, as it sets point the dish 90 deg anti-clockwise from the sun.
Hope this helps