Drive it in any gear at 3k constant (no accel or decel) and it's quite lumpy / almost stuttering - car has only been run on Optimax since i've had it (1 year and a bit) and always warmed up properly - recently had an ECU reset - any ideas?

 

Come over to mine for a "wasting away" session and I can do a DD logging session

 

Yep, problem is Optimax, try a tank of SUL for every couple of Optimax tanks you use.

 

How odd - mine had the opposite

Ran on 95 RON for few months (Sport before anyone gets carried away), and the car felt like it was getting clogged up - hesitation particularly between 2.5 and 3k. Changed to Optimax (for months now) and it's smooth across the range.