Has any one used Y10 to clean the inside of their boat?
Its basically a premixed oxalic acid, fibreglass restorer.

Ive used it in small quantities to remove those little rust stains you get, and the results are quite impressive.

Any thoughts?

this be the stuff

I've always used oven cleaner or G6 polish on rust/stubborn stains with good results.

That stuff looks like it should be quite good, and may also be good for the horrible yellow colour that the white boats go underneath. Not expensive either.

Let us know how you get on!

Y 10 certaily works on rust stains on dacron sails - eventually - the oxalic acid is very dilute so it doesn't harm the cloth but it can take more than one application. I used it a couple of years ago to improve the look of a few older sails that I got with my first Merlin. 
Best of luck.