When i finally got round to fixing the problem, i bought my brand new tank. After much swearing and force i dropped the old tank out without having to remove the seatbox.
Just the 3 bolts holding it in and all the auxiliaries connected to it. Its definitely worth using a car jack to support the weight of the tank even after its been drained. It also reduces the fall when you hit it from the top, it did need a lot of wiggling and repeated attempts admittedly.
Once out the new tank popped in relatively easy, definitely a lot quicker using the jack to push it up. One foot on the jack and 2 hand pushing the top across as it went up.
After all the hassle and my late night I found that the cause of the leak was a missing screw in the sender unit as it hasn't leaked as much as it was. Oh well, good excuse to fit a new tank fitted and holes plugged.
The truck is a 1993 200tdi 90 in answer to Maverik.