This is a bit of a grey area - it is only illegal to mix cross ply and radial tyres or fit tyres that do not meet the size or rating requirements of the car. There is no law that specifies the same brand of tyre needs to be fitted on each corner, but it will affect the way the car performs, particularly in the wet or under braking.

Was the car purchased from a dealer/independent garage? You may possibly be within your rights under the Consumer Rights Act to reject the car within 30 days of the sale, but this is pretty unlikely and the dealer will no doubt dispute this, requiring legal advice on your part.

Sadly, as this was not noticed before the car was purchased the cheapest course of action would likely be to replace the tyres, and treat it as a learning curve to check before buying (and avoid said dealer!).