Most UK registered Seneca II's are certificated at 1999kg, so don't despair just yet. Provided you haven't got the one with the long range tanks then 4 up plus full fuel is almost possible. Even with the long range tanks there's normally not a problem.
However - with lots of fuel and only two in the front you're W&B is seriously out, so needs careful consideration.
Drop me a PM if you want to discuss further (all my Seneca flying has been in the II)