I have fitted the seat back insulation panel (great condition) but whatever was under the seat was in fragments. I need pointing at the correct parts(s) to buy before fitting the squab.
The original item was glued on and may also have covered the narrow area between the floor and the seat base.
As I have the full rubber floor mats I need to know how this gap is finished.
Guidance will be gratefully received.

The same stuff I think (well it is on mine). Mine too was stuck down and although it could be re-used I shall be buying new. Seen some on van der laan and its all labelled as insulation under seat. The bit between the seat and floor is a thin piece of black plastic that clips under the seat. Again this is on van der laan. Let me know if you need some photos

I made my own insulation Peter out of the stuff used on roofs for insulation, as you may recall. But it was covered with a foam cover that wasn't original. I did try and put some behind the seat back but it too lumpy to fit it all in.

As Damian says there is a little cover you can buy that covers the bit between the squab and the floor, with a cut out for the tunnel.

That bit betwixt seat and floor..is it just a piece of black plastic sheet because I do have such an item in good condition. Just not sure if it went under something else or had another item above it.

Yeah it looks like that Peter. It looks like vinyl or black plastic that has been sown/trimmed for a smooth edge.

I am sure you could make something that with a couple of slots in it so that it can be secured over by the brackets that the squab clips down onto. Then tucked under the floor mats where they meet.

This is the item that Axel Gerstl sell it appears to do both jobs insulate and cover the bit between the squab and floor. A bit pricey for what it is. I am sure you could source a nice big fat carpet underlay off cut from your local carpet shop. It's not like you see it under the seat.

https://webshop.fiat500126.com/katalog/artikelinfo/2828/insulation_mat_under_rear_seat_custom-fit

DSC_6136 by peterthompson, on Flickr
I like that frugal idea Tony. I also found that one old photo shows where the vinyl bit goes. On my way with that one. Thanks.