Thanks Mike, just had a look and repositioned mine, something nice and easy to sort out for a change! Took a few pics too, mine is the W204 so not all will be the same obviously, I would imagine some are similar though. If anyone is interested here you go:
The black plastic clip (near the interior light) has to be removed to pull the boot lining away to gain access to the spring on each side.
To remove the clip, the small section that passes through it (expanding the plastic 'pin' on the reverse side, locking it in place) needs prising out first. Flat blade screwdriver did the job.
Expander section removed.
Plastic clip removed, boot lining can be eased back to gain access.
There are 5 positions for the tension spring (W204) mine were in the second from the front, in this position when I bought the car.
I moved it one position back, to the middle setting, for a bit more tension. This did the job nicely, boot lid springs open completely now.
As Mike said, all you need is a good pair of sturdy grips and it is easily sorted, to move the spring position.