Chain was indeed a bit too tight. After adjusting it properly the vibration got much better, although it is still there.
I had a look at the front sprocket and it is at the very end of its working life, which is likely to be another source of gear lever vibration.
I am impressed by how quickly it is gone! The bike has 8.5k miles, the chain seems to be fine with no notches and little play when pulling it at the back of the rear sprocket, the rear sprocket is also in very good shape and the bike always had scottoiler.
I am not a big fan of changing just one component of the drive chain assembly, but this time I will give it a go and buy only a new front sprocket.
Thanks all for your help and Ill get back when I change the front sprocket and hopefully sort the lever vibration.