"I am going to do an oil and filter change tomorrow as the oil is very black so needs doing anyway but thought the old oil might be causing a problem with one of the hydraulic tappets, whats think?"
Yes, dirty oil might cause the ticking too. We usually recommend 3 months/3k mile oil change when using conventional mineral oil.
"The ticking gets faster as the revs get faster and it DOESN'T do it when the engine is cold."
It could also be a pin hole leak on the exhaust system gasket or manifold if it is a ticking sound which goes away with temperature change. But change the oil first.