My rough calcs so please feel free to shoot them down:
SR4 dry weight (600lbs) = 275Kg
Outboard - unsure what you have but say a 50hp = 120Kg
50 litres of fuel = 50x 0.74 = 37Kg
Onboard electrical (inc battery) = 12Kg
Onboard others (anchor, wrap etc) = 15Kg
Total = 460 Kg
Assuming the towcar can tow 750Kg, that leaves 750-460 = 290 Kg for the weight of the trailer plus error margin.
If the tow car can only tow 500Kg, you may only have 40Kg for the trailer and error...
Does this look correct?
EDIT: Legal max as Trailer Guy mentions is 750Kg unbraked https://www.gov.uk/towing-with-car