What is covered under each service is detailed on Toyota's website - https://www.toyota.co.uk/owners/service-mot-maintenance/servicing - and this has been the case for a number of years. I've had service plans on each of our last three Toyotas, and have been aware from day one of each service plan, which services were included. A plug change is included as part of the Full+ service, but if your service plan was only for Intermediate and Full services, a plug change would be extra.
To know what one is paying for, in my opinion, is an essential part of any purchase.
Aside from the above, as your car is over five years old, you could have taken advantage of Toyota's Essential Care scheme, which reduces the servicing costs quite substantially - for the Auris £120 for a silver (intermediate) service and £200 for a gold (full) service. Also til the end of 2019, there is an offer of a silver service and MOT for £99. See the link above.