Finn
Do check the idle speed as that is possibly the most important factor , the correct oil will prolong the life of the box but would not affect your issues, however the idle speed is a likely cause. While you are setting the idle speed have a good lock at the HIF6 SU carbs, as they have known issues around the mixture adjustment lever.
If the worst does come to the worst then a new clutch is definitely an engine out job. This is a big job in every sense of the word, so try the simple fixes first before contemplating a task like this.
