Over the last few weeks I have been noticing a wierd scratchy kind of sound in my left ear which seems to emanate from the corresponding FHole. (no jokes about my playing please!)

Now and again I have occasion to work in a noisy environment (usually my colleagues moaning about how much work there is to do) so I have access to the industrial squidgy type of ear plugs.
If I bung one in my left ear, my playing seems a little clearer.
should I continue doing this or will it be detrimental to me when playing in a hall with accompanying instruments, eg next Saturday's recital?

Beethoven may have gone deaf, but that doesn't mean he was actually ok with it.  It certainly won't make life easier for a composer or musician and not being able to listen to music would suck.  You want to take care of your ears the best you can.

I have some hearing loss, but it was more due to ear infections as a kid than even the louder bands I was in.  Some of it was no doubt due to loud bands, though, before I got some sense about it.  A musician, even an amateur or the ones who say "Oh, I only want to play for my own enjoyment" should not let ear infections go untreated.  They can cause scarring of the eardrum pretty easy and are not something to try and "tough out" like a sore throat.  Get to a doctor, get some antibiotics, and get rid of them as fast as you can.

Other than that, it is good to use some common sense.  If your ears ring after a session or the world seems oddly quiet for a few hours, it was too loud.  If you can feel the sound in your ears, even if it is not loud enough to be painful, it is enough to cause damage over a fairly short period of time.  Hearing protection like earplugs are a good idea if it can't be turned down quieter.  Use speakers when you can instead of headphones or earbuds.  It is very easy to run headphones loud enough that if they were speakers somebody would call the cops.  That's a lot of sound to have awfully close to your ear.