Not sure where a air pocket issue was ever raised in the posts?
These Megaflo HW cylinders accomodate the ~ 3.5L of expanded water by having a air space, commonly called a air bubble in the top of the cylinder, others, the most common type of HW cylinders use a external expansion vessel to accomodate the expanded volume. This air bubble must be re established because the air is slowly absorbed by the water, this is why a plastic air separator/baffle is used to extend the frequency of re making the air bubble, I wouldn't expect it (the bubble) to last longer than a year, if its not then a expansion relief valve will lift and expel up to 3.5L of water each time the cylinder is reheated if no HW is being drawn off at the same time, you can see this in a tundish from the waste pipe from the expansion relief valve. You should also notice a extra spurt of HW in the morning when/if you open a HW tap.
Megaflo appear to be very receptive to problems with their cylinders, you might ask them if you were to mail on a sample the plastic bits, would they analyse them and if theirs then I'm sure they would offer some remedy, probaby a new cylinder, you could also ask them what would be your cost if the analysis proves otherwise.