Dismantling base of utility room under sink cupboard?

We have a leak at the point of entry under the tiles, it's deep down, and the plumber needs to access under the cupboard under the utility room sink.

Ideally, I would remove the base of the unit (represented by the blue cross). However, it appears that it might have been built before it was dropped in. I can remove the corner post (green arrow) which has two screws. But there are no screws at the left side of the unit at the base. Looking underneath at the corner (red arrow), I can see a screw (2nd picture) that I assume is gripping something from the end, IKEA-style. But if I loosen it, there is no play in the system for it to pop out. Also, the other end is extremely awkward to access, though I should be able to reach the corner screws from underneath, hopefully.

Do you have any tips on how to remove the base? Yikes, it sucks. The easiest option might be to cut the bottom in situ and install a new base with visible brackets.