Fuse bloing on power hose?

I have a KEW 240v Power washer, which is approx 20 odd yrs old. It has always worked 1st time on the odd occassion when it is taken out for use. However, now it will only work perfectly so long as you start it with the trigger of the lance depressed, and only switch it off and on with the main on/off buttons. The problem is that it continues to blow the 13amp fuse everytime you release the trigger on the lance. I have run it without the hose attached and it works, I have run it with the hose attached and it works. I have replaced the oil in the pump and made sure that all internal workings are free from rust and dirt. The pump is of all brass construction and the motor is perfect.
The lance has been thoroughly cleaned and works perfectly when cleaning the slabs but as soon as you release the trigger to cease work the unit strains momentarily and immediately blows the fuse.
I have used it via the HD extension cable and direct from the socket which excludes any outside problem.
The problem seems only to occur when the trigger is released. The unit works perfectly but being unable to release the trigger and having to switch it manually at the unit, as you can guess, is driving me potty....