The bit that intrigues me is; "light weight blocks from a previous build"

My guess is these are blocks you would normally use in the"inside" walls of a building and as such the won't be frost proof!

Plus they are generally porous and won't be water tight, meaning you would need some form of lining!

The other thing is; they would be permanently wet meaning this would add to the frost proofing problem.

So al in all from a " build" point of view they would be OK but for the reasons given above, I wouldn't recommend it!

thanks, they are leftover from a wall we had built in the front garden and I would be using a pond liner and rendering the outside of the blocks. So in theory should be fully watertight. My main concern is would they be strong enough to hold the water needed for the pond?

In that case you could strengthen the walls by building pillars in each corner and half way along each long side.

You can buy an additive to waterproof your rendering, it is normally added with the mixing water!