Thanks for the replies.
Not had the bike long so I don't know how long the crack has been there.
I think, as suggested, it may be wise to just leave it at the moment and keep an eye on it and see how/if it progresses and then deal with it.
John, you made a reference to bronze inserts?
I was helping a friend with his SR over the week-end (who has a crack on each side) and we noticed that there were bronze inserts and assumed that they had been done at a later time as a repair? Were they fitted as standard during the manufacturing process?
Andy