On the Y7 admissions page they have a link to a document specially about SEN arrangements and what is required as evidence for the 10% extra time.
http://www.saintolaves.net/uploads/cked ... r_2019.pdfLooking at the compare schools data they have 0.2% of pupils at Olaves with SEND so a very tiny proportion but not dissimilar to Newstead where they seem to deal well with those needs (DD has a friend with a physical need and they work well to accommodate her needs).
The number of people who therefore have children with a special need (there may be others with needs but not registered ones) at the school will be very very tiny (two children in fact!) but hope someone comes along with actual experience of having a child with Dyslexia at the school but it maybe as the numbers are so small they may not exist with similar needs to your DS. Probably best to context the Senco at the school for an informal chat.