Strange object inside Sill section..

[shanee]

Hi all - I'm starting to cut away a section of rusty sill on the rear drivers side of my pre-facelift Rover and i'm a bit confused (not uncommon). There is a section right up against the jacking point that is not steel and is made of a soft glass fibre type material with a stiff backing, about 1cm thick (see pics). Is this meant to be there, i'm worried someone has been here before and this is some strange bodge. If it is, it has been done very carefully and very neatly. I'm hoping someone familiar with this area can confirm whether or not i should rip it out as i am worried it may be flammable when welding. On the other hand, maybe it is meant to be there, anyone have any ideas...?