Yes, your point is well made and I shall take the boat + trailer + representative gear down to a weigh-bridge near here before I buy anything.
The replacement is a bit complicated because I will have to cut the old suspension units off and grind a smooth patch and get new plates (with suitable holes) welded back on, the tube the old suspension units are pressed into was welded to the frame before it was galvanised. Rapide have gone out of business and West Mersea who nominally took over the business do not provide replacements of this model. which dates from the ’70’s
All eminently solveable (soluble?) but I wanted to draw on the vast experience represented on this forum before I get my angle-grinder out.