Thanks all.
So, as a starter I thought it worth changing the spark plugs.. fairly straightforward and as it turned out, they looked pretty ropey anyway.
Turning on the car, I instantly thought it was fixed, however, the noise gradually reappeared. Don't think it was as loud though. Definitely from the head of the engine.
I left the car running and after a few minutes, and as the car got warm, the noise did disappear. Ran smoothly after a short while.
So.. could it be that as I haven't used the car for a few weeks, there was a lack of oil throughout? Not sure. I wasn't intending to use the car until at least March, so it will stay in the garage SORN'd until then. I'll then get it out and do a proper test when I can use it more regularly.