Thanks to all for your support and advise. I ordered the groomets on Sunday using the suggested link. They arrived today (Tuesday), and I popped them in at lunchtime in just over an hour. The old grommets were a bit tricky to lever out, but once I realised finesse wasn’t part of the job description I managed okay. The near side grommet seal was completely shot so I am very comfortable the problem has been correctly located and rectified.