they are incredibly expensive
I went through this a few years back - see //www.diynot.com/forums/viewtopic.php?p=1235388#1235388
Unless you can find a friendly heating engineer, I would be tempted to try a supplier of "O" rings, where you should be able to buy about a thousand for the price of one from Pegler.
They should be ashamed of themselves for their wicked profiteering.
I took them all apart, descaled, polished up the brass with a green nylon scourer, reassembled with silicone grease, and PTFE tape on all the threaded seals. If you're in trouble, wrapping PTFE round the spindle until you can get the "O" rings may stop them leaking. You may find it extremely difficult to dismantle them without a vice, some big ring spanners and a socket of exactly the right size, and possibly a flame.
Still availlable !!
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local