I ended up looking the service history I have for the bike and it looks like it hasn't had a new chain and sprockets since 2006 so I figured that was a good enough reason to have them replaced - booked in on monday with Morpheus in Bristol 
Edit: Just as a final update, the new chain and sprockets have solved the knocking and the bike feels a lot nicer though I did get caught in a rain storm :p

Edit: Just as a final update, the new chain and sprockets have solved the knocking and the bike feels a lot nicer though I did get caught in a rain storm :p