The mileage has just clicked over past 110k so she's no spring chicken but in some way I prefer a car that has had some use rather than sat still for long periods.
I was trying to figure out the thin, thick oil debate. When the car first started making the noise it had cheap oil in it, correct grade but seemed thin & watery (I think it was a brand such as Comma) so when the noise appeared I used a good quality engine flush then changed the filters, plugs, distributor cap and oil to good quality oil (Morris). I also added a couple of cans of hydraulic tappet treatment but it has had no affect.
I had a good listen to it earlier using a piece of pipe as a stethoscope and I'm pretty sure the noise is coming from the right bank and from the rear of that (closest to the driver) so I think I'm going to remove that cover when I get a free weekend and have a good look. The noise completely disappears when the rpm increases then returns when the engine ticks over of is at low rpm.
If I remove the valve cover, please can someone tell me how can I tell if it's a sticking tappet?