Dont Frett to much Danny, as you know a Murmur is basically just an extra sound. I had one diagnosed at my last medical (stress induced at the examination) The CAA are very very good, they will give you a good idea of whether or not it will affect a class 1 initial. Talk to your GP, get the info, and forward it to the CAA for consideration.
Unless it is relatively serious, you should be fine
Hope this helps
Cheers
H