Rog, if you are going to be pulling the saph on a trailer, then the first place to start is weighing the Saph or read the chassis plate.

Without a trailer test:
If this is, say, 1200kgs, you are left with 2300kgs. (3500 - 1200)
The trailer, say, a Brian James A-Max is approx 500kgs. That means you are left with 1800kgs. (2300 - 500)

1800kgs is the max kerbweight of the tow car.
Kerb weight is a discussion point on it's own, depending on when the car was made!

Just to give you an example here, my Mondy is approx 1450kgs kerb weight. That would mean that you would be leagally allowed to pull the Saph on the trailer with your licence.
Max train weight = 1450 + 1200 + 500 = 3150kgs. Leaving 350kgs for people, tools, etc.

However, the Mondy max tow capacity is 1800kgs. The trailer and Saph totals 1700kgs.
The recommended tow capacity of any car is between 80-85% the max tow weight. Using the example above, my Mondy pulling the Saph on a trailer is at 94%.
The recommended tow is down to driver experience. I'll happily tow higher than that, but I've been towing for years!

If you got a heavier car, which has a max tow wieght of say 2000kgs, that would bring you into the 85% mark.

The next thing is the kerb weight of the tow car. If you entend to take tools, people, etc, you will be limited to a car that weighs less than 1800kgs. That is a car with a kerb weight of say 1600kgs, a passenger at 80kgs and 120kgs of tools, crisps and beer!

If you got a 4x4, that weight including the trailer and Saph may bring you over the 3500kgs max trian weight that you can leagally pull on your licence.
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Or you could just take the trailer exam and go an buy a 4x4 / Transit!