using top coat plaster to stick plasterboard

[almostanexpert]

I've got a piece of 9.5mm plasterboard (only 3' by 2') to stick to a brick wall.
Instead of spending a fiver on a small bag of plasterboard adhesive, can I use a bit of top-coat plaster instead?

 

[noseall]

For a piece that size i would say yes, but use lots of gear and a good splash of pva.

I presume it will be 'locked in' by surrounding plasterboard....?

You can always add a couple of fixings once the muck has set but i doubt this will be necessary.