eventually tesco got back to me
<< Thank you for your patience.
I have checked and can confirm that "99 octane" unleaded petrol has a minimum RON of 99 and a MON of 87.
It exceeds the British Standard Specifications for BS7800 - superunleaded fuel, and BSEN228 premium unleaded.
It contains a high level of a detergent additive which exceeds the level of performance specified by Major European Motor Manufacturers, to help keep engine fuel systems clean.
I trust this information is helpful to you and I would like to thank you for your enquiry.
If you have any further queries please do not hesitate to contact us at customer.service@tesco.co.uk quoting TES830216X.
Regards
Deepak Kumar
Tesco Customer Service >>
this gives Tesco 99 octane a rating of 93PON
shell is 98ron (98.6 typical) 86mon (87.5 typical)
which gives optimax a rating of 92 PON (93 typical)