Zodiac rowlocks often fail. They're simply not up to the task. Fine for occasional rowing, but as soon as you put your back into it (which normally happens when the wind gets up or you're rowing against the tide) - then they disintegrate. The oars themselves, assuming they're the wooden ones and the stainless steel rowlock are excellent, it's just the rowlock bush which is seated in the rubber mount which causes the problem. The stainless steel bush is too lightweight. When rowing it flexes in the rubber mount. You could try removing it, part-drill or cut a larger hole in the rubber (careful with this bit) and insert a new larger overall diameter with the same internal diameter stainless steel bush which is bonded in place.
Not sure what else is on the market you could replace it with?
Is that with or without VAT?