Don't have higher than 97 in Northern Ireland

 

That sucks

 

I would say it's unlikely that you've damaged anything, especially if your car is pretty standard. The Subaru ECU has pretty robust knock management and uses several approaches to managing detention; reactive, learned and overall, in simplistic terms.

Probably what you're feeling is the ECUs response to knock seen in different driving conditions (learned), or maybe even the overall which affects performance across the board, if it's seeing enough knock.

If you start running it on Vpower or Tesco 99 again it will take maybe 30-40 miles of different driving to reset the various metrics it stores, or you can force the ECUs hand by resetting it. Remove the battery, pump the brake a few times and leave for 5 mins or so, then reconnect everything. Afterwards I would suggest doing some town driving, and light accels on the motorway / dual carriageway, so the ECU can relearn it's fueling before opening it up fully again.

 

You forgot the clicking your heels three times and saying hail bloody Mary whilst standing on one leg...

 

more like "Hold onto your butts...."