Thanks everyone.
I've ordered some of the Wynn's stuff for now, just to see if it does make a (marginal) difference. I'm pretty sure my tappet noise is due to the POs lack of servicing - its pretty nasty in that engine, judging by what I can see through the oil filler
The garage that I bought it off did an oil change before I got it, so I didn't get to see what it was like, but I suspect it was... well overdue!
Hopefully, some decent cleaning agents will break down the sludge and allow the tappets to operate correctly again. I'll report back after I've done a few 1000 miles.
Thanks again