Thanks for the responses and advice.
As I mentioned earlier the Gearbox oil was changed in April this year; I have emailed the dealer who serviced it to ask what oil was put in. If it is anything other than that which is recommended I agree the first thing to do is change the oil to the recommended one. I would then see if that made a difference. I have also asked what the additive is that the dealer is proposing to put in......I will let you know the answer when I have it (hopefully not sawdust!!).
