Update on the situation, I changed the gearbox oil to silkolene Silktran syn 5 and the crunch when shifting to 4th gear has gone however there is still a crunch when going to 5th. I think the 5th gear crunch is down to what Ciaran has mentioned, the loose nut within the gearbox so I will have to have the box opened shortly to investigate hopefully no further damage has occured due to this.