An air leak between the lift pump and the bottom of the tank dip tube inlet filter is most probable as others have explained. A piece of clear plastic tube in the line will allow air bubles to be seen which can help in tracing and give some reassurance that you've found the problem.
Time for a new one or have it welded. There's clearly very little metal left.
If it were less worn it could have been repaired with a bush but that's really too far gone.
The drawback to the travelhoist is its very wide. Boats must be spaced at wide centres making poor use of available yard space. The tuningfork type of lift allows boats to be almost touching each other. Of course when owners want to work on their boats in the yard that can be a problem...
I asked the MCA today whether our club, Royal Northumberland Yacht Club, was exempt. They told me to ask the club.
What's the penalty for putting the initial letters of the club on the transom rather than the port of registration?