I've developed marks on my neck from your wetsuitUpdated 14 days ago

This is a very common occurrence due to the material rubbing against your neck while swimming. It is not a fault of the wetsuit itself. Once you add in the movement of your neck and head as you breathe while swimming, this can lead to some painful spots on the back of your neck. 

You can help prevent this by ensuring you apply anti chafe balm at the back of your neck where your suit sits. 

You can also be aware of your head position in the water; a higher head position compresses the skin at the back of your neck and onto the neckline of the suit. 

Saltwater is also more likely to create a reaction due to dissolved salt in the liquid, so you may need to apply more balm in these conditions.