The seat on my NC30 is only held on by fixing as the other has broken as they are only made of plastic.

How do i fix this. Is there a trick of the trade or a kit or something...................

Mine was similar, didn't bother fixing it till it failed the mot for it but there are three options afaik:

Buy a new tail unit/one side of it at least.
Get a plastic weld/repair kit thingy and re-build the fixing bit.
Or do what I did, buy a nice big washer so that when you bolt up the seat the washer bends into shape and then acts as a clamp on whats left of the fixing, worked a treat :)

thanks. washer sounds good. will give it a go on sunday.

If you want to reform it, try using Plastex. It's as tough, if not tougher than standard plastic, and you get enough supplies in a small kit to make hundreds of lugs etc. Worth buying.