Thank's guys, the difference between a tick and a knock is small to the ear but large to the pocket as I understand it! My heart is telling me it's a loud tick, but my head is saying knock!

With the engine at idle, I disconnected one coil at a time to see if the noise went away (if so - indicating to me that it would prob have been a rod bearing) it did not - phew!

I will try the flush/oil change thing and take it easy to see if that works! Am I going to break anything if I keep (calmly)driving it though?

I would be a little careful.

But if it's a stuck lifter, it shouldn't do too much harm though.

Just don't use it to race on a track or drive thousands of miles with the engine ticking like this.