Hi chaps, sorry for not updating sooner!
So yes, the service centre confirmed that the service history was correct and that the mileage was 75k+. They couldn't actually confirm the exact mileage from the ECU, however there were some historical data capture points with mileage (such as the 12v battery replacement) which match up with the service history. They suspected it could be a replacement speedo, however they couldn't confirm that without taking the whole dashboard apart. They gave me detailed print outs of the service history and the hybrid health check certificates.
I've been in contact with the dealer to understand what my options are. I have more than enough evidence to just get a full refund, however I'm considering just taking a partial refund based on the higher mileage. The car itself is in great condition, which the service centre technician also commented, so I'm reluctant to have to hand it back. That said though, if I do take a partial refund it will need to be a significant one. As of now I'm still waiting to hear back from them, although they have acknowledged and accepted they have farked up on this.