Thanks to Multisync a great idea for modifying the seat to make things more comfortable!
I'm not sure if the Toylander I seat setup is different to the Toylander II, but I had to modify things slightly to get it to work. I could not tuck the bottom of the seat back in behind the seat base to stop it moving. In this first photo you can see plastic blocks I added to sides of Toylander to form a slot for the seat back to rest in. Plastic blocks were designed to be used for putting together flat pack kitchen units, and came from the local DIY store. The rounded edges of the block make it easier to insert the seat back. Original rubber stops hold the seat back upright.

Here's a picture of the seat in place, with foam padding added to the seat. Seat base foam is loose, but I intend to hold it in place with Velcro once it has been covered.

Next is a picture of the seat laid flat for an adult driver. We used two layers of foam on the seat base, but only one on the seat back. This means that when the seat is laid flat, the entire seat is level.

Here's a picture of the rear of the seat. You can see the plastic blocks, at the base of the seat. I'm hoping I can paint them to blend in.

Now we just need to get the seat covered. Over to the missus!