Made some more progress on the Tiran 5. Trumpeter have some interesting ideas about models...they make both their motorised and unmotorised versions have gearboxes and battery holders. The gearbox has a screw on the underside, which can be unscrewed, to remove the gearbox. Trumpeter then give you a cover for the resulting hole (the oval shape in the picture). However, you need the gearbox, because the axle is part of the gearbox! Lucky, it can be removed (although, I wait to see if the model still holds together...!).
More interesting is that the battery case has a switch attached to it, which comes through the rectangular hole, and cannot be removed without some force. Also, there is no way to close off this hole, even if the switch is broken off. So...I finally found a decent use for a Minicraft kit (actually, the flat part of the sprue used for the name plate), yes, it filled the hole, will fill to top.
There are also two depressions with towers in them on the underside, which I guess are something to do with the manufacturing process - can anyone let me know what they're called?
Anyway, it's all been filled in and sanded. Here's a rather poor picture of how it looks