How best to cut this staircase side?

[Flipjango]

We have a very large under stair cupboard - its hard to get to the back of it. I'd like to cut down the side where I've marked on the photo, to create a smaller cupboard at the bottom end.

Everything will be painted after I've finished, but I still need to be super careful to make sure that I don't damage anything. Any suggestions how best to cut this to ensure a perfectly straight edge? I think the middle section of trim (the right hand side of the triangle) can be removed, cut with the track saw and replaced, but the horizontal section to the bottom cannot and neither can the diagonal piece. I have a track saw, jigsaw, mitre saw and a mini angle grinder as well as hand saws.

 

[foxhole]

Only a tiny space there hardly worth the effort.

 

[freddiemercurystwin]

Perhaps but when you don't have a lot of storage space every bit counts.

 

[Tigercubrider]

Tracksaw with guide

 

[chirpychippy]

I wouldn't use a track saw and guide on that vertical face, using a track saw vertically is dangerous, one slight movement and the blade will bite and throw the track saw out.

Dare I say it, Use a multi tool and a fine blade against a guide and go slow.

You'll have to remove the bead that closes the gap between the panelling and the bottom of the string aswell.

 

[theflyingpig]

Second that... Multitool is your best bet... I'd screw a batton on to give me a guide to cut along.....

 

[freddiemercurystwin]

Then fit a handy trim/architrave to hide the shoddy cut!