Ended up geting hold of a couple of front hubs. The reluctor rings on mine were pretty good, but the hubs I got looked a better.
Replaced both in about 1.5hrs and took for a run on the motorway - all good and the speedo needle did not flicker either.
What did surprise me was that the road noise was a lot less, before I just throught that the state of the roads etc added to the noise through the tyres. But I really think it was the bearings.
When the orginal hubs were on the vehicle there was no play, they would rotate without any issues etc. I checked them when I got back and there was no sign of wear/failure and they spun without a single sign of hesitation or grittiness etc.
So it goes to show that merc origianl parts (well the bearings) can last (264500 miles) and will not just fail without notice (which I clearly didn't)