Hello Johnny, welcome to the forum. The Speeder Bike is pretty common and towards the lower end of boxed vehicles. The plastic tray is kinda cool because its the only plastic insert in the vintage toy line (except for the Early Bird tray, the very first thing Kenner produced), but both plastic and cardboard seem to appear in similar and large numbers and so I don't think one appears to be rarer or more valuable than the other. In the case of most boxed toys kids would pull the toy and inserts out together, and the insert would be fiddly to put back in the box and so they got chucked out along with the xmas or birthday wrapping paper. However I guess the Speeder Bike insert being designed like a tray, slides in and out the box pretty easy so more of these inserts seem to have survived proportionally to others in the vintage line.
That said, what you describe with your example also having bagged contents does make it more valuable, being a never assembled 'never removed from box' example. Pretty tough to value an item without a photo (you can always look on Ebay completed listings to see what others have sold for), I'd say in the Speeder Bike case the bagged contents ought to add £10 to the value.
