Block paving does not sink. It spreads.

A block paved surface has no strength of its own in the way a slab of concrete does. It works because every block is wedged against its neighbours, and the sand in the joints locks the whole field together. It is closer to a dry stone wall laid flat than to a solid surface.

That works perfectly as long as every block has neighbours. At a free edge, the outer course has nothing on one side. Every time a wheel runs near it, that course creeps outward by a fraction of a millimetre. Over a couple of years it adds up to a visible gap.

Once there is a gap, the field is no longer tight. The joint sand starts to wash out, the blocks move against each other, and the middle settles down into the slack. That is when the customer rings up about the dip in the middle, and that is when the base gets blamed for something the edge caused.

The fix is a concrete haunch: a wedge of concrete run along every free edge, under and behind the edging course, before any of the field is laid. It is half a day of work, it is entirely buried, and no photograph will ever show it.

When you are comparing quotes, that is the second question after the depth. How are the edges restrained. A firm that says kerb edging on a concrete haunch is describing the job. A firm that says we edge them properly is describing nothing at all.