Ok, here is a real quick pressure test for the OP. Engage your Emergency Red Handle and manually open and close your top about 3 or 4 times. Is it getting easier to move each time you do this, less and less resistance when going down? If so, it's because your draining out any remaining fluid from the system. Now bring the top to the up position and reset the red handle to normal. Push top down and WITHOUT using the emergency red handle attempt to manually drop the top. If there is still stiff resistance then it's possibly a jammed by-pass valve in the pump. However, if it moves freely, then your hydo system is no longer pressurized and it's either a bad hose or hinge lifter.
Again, You folks in the UK are really lucky having a number of folks willing to do thru the trunk relocates. One of these guys should be able to pull the motor thru the trunk and ID thru the plastic top if your low on fluid. I'm hoping it's simply a jammed by-pass or a blow line that can be changed out thru the trunk. The trouble shooting info I'm providing was gained the hard way