Car: FN2 CTR & Si both Milano Red
Join Date: 16th May 2017
Location: East Sussex,UK
Thanked 66 Times in 66 Posts
Since you say this issue has only just come on I'd go for the fuel first if you have put supermarket crap through her recently.
If you can't get Shell V power then use BP Ultimate or Esso synergy+.
These premium fuels have a higher octane rating so give more burn and so more poke at lower revs and that may be what you have just noticed has disappeared.
Valve clearances at that mileage would be a good idea since if they are off it won't help - the later FN2 interval is every 62,500 miles, the earlier ones every 25,000 - either way treat it as overdue.