I think you will have to consult the manufacturers to get permissable towing weights if there are no plates on the vehicle. However, have you thought about legal technicalities? The Cabstar is under 3.5T, so you can drive it on a car licence, but don't forget you must have a B+E to drive with a trailer on. The combination will be over 3.5T, therefore a tachograph must be fitted and I think you must hold an Operators Licence (as you are working for hire and reward). If the braked trailer weight for the Expert is 1968, I think it will struggle with 2600kg (and could well be illegal). As a guide on the Cabstar, it is usually recommended that a trailer should be no more than 80% of the towing vehile weight and you are almost on the limit for that. You may need to buy a more suitable towing vehicle.