Re-levelling linear drain

We had a 100mm linear drain installed by a reputable contractor a couple of years ago. Unfortunatly the drain doesnt fall towards the outlet, so we have alot of standing water in the drain. Its been a couple of years so its probably to late to appraoch them to fix it. Is there any easy way of relevelling the drain? I was thinking along the lines of either:

Pooring some form of waterproof compound / bitumen at one end to at least get it level/ slight slope.

or

Sanding the bottom to the required gradient then installing guttering on top and sealing.

or

Renting a jack hammer, removing the existing and starting from scratch.


Thanks for any advice

Moo

Dont do anything but post pics.