Most clutches are self adjusting through the use of springs, so it is possible the clutch has worn out, removing the sprocket and inspecting the shoes would verify this. It could also be a worn sprocket, check the sprocket teeth, but if they look good it may be the clutch is worn. Some models have a replaceable sprocket if its just the sprocket others the sprocket is part of the rim and clutch. Large logs are not has hard on saws as the smaller green limbs. There are many places that offer parts, but eBay UK may be your best bet as it was a popular design not unlike many Homelite and MacCulloch models of the same era.