Have found a nice Sport (estate) relatively local to me for about the right price. My only concern is that I was looking for a car with 60-80k miles on it, and this has just over 100k.
However apparently early this year it developed a knock (sounds like piston slap) so they had the garage check it out.
One piston was worn, so they got the garage to fit a new block, pistons, cambelt, main bearings and clutch.

Now my real question is would that make the engine near enough as new, or if not how many miles do you think it would knock off?

Or I spose more to the point, if I end up with a choice between this car or one with 80k on it, which do I go for?

The better car;)

Just because a car has 20,000 less miles doesn't make it a better car. Bad servicing and neglect would easily negate less mileage IMO.

Well there is that I spose.
But would that work make sufficient difference to the engine to make up for 30k miles (assuming engines had been driven the same)?