Hi Dave,
Cut End is the north side (Freiston side) of the Witham Mouth. It is accessible from Freiston Shore, but it is quite a hike along the seabank from the main car park.
There is a much smaller car park at the bottom of Cut End Road (drive into Fishtoft, turn down Gaysfield Road and cross the Hobhole Drain). The car-park is at the bottom of the sea bank. Please note - I've never had any trouble parking in here, but I certainly wouldn't leave any valuables in the car either! Anyway, take the steps up the bank, turn left and Cut End is about 400m away. You'll have the river on one side (obviously) and some bushes along a set back fence line on the other, which are particularly good for thrushes in Autumn/ Winter.
Once you get to Cut End you'll see a two storey hide. The top story is for members only, but you can take shelter underneath and scope from there. You'll have views right across the river mouth to your right, the Wash in front and Freiston Shore on your left.
Alternative, if you visit Frampton Marsh you can walk through the reserve and get up onto the south bank of the river and walk up to Tabs Head - which is pretty much directly opposite Cut End. Again, it's a bit of a trek (approx. 3km) and there is no shelter waiting at the end, but whilst it can often be a very bleak, lonely and quiet walk, the bushes along the path have occassionally been known to hold a little surprise at this time of year (Wryneck, Barred Warbler, Firecrest, Shrikes, Redstarts, etc). It's definitely worth checking out.